I hope you managed to see the planets Venus, Saturn and Jupiter all in a line just after sunset on one of our clear nights. On Sunday 28th November I spotted Venus bright in the sky just as the sun was setting from my armchair in my lounge! I did have to get up off my bottom to spot the other two as the skies darkened although Jupiter is so bright that you cannot miss it either.
The following chart represents the night sky at 11.00pm GMT on the 8th of December and at 10.00pm GMT on the 23rd December. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart. If you are observing a little earlier in the evening then the view is shifted 15 degrees eastwards for every hour before the specified time.
This month the focus is on the constellation Orion- The Hunter. It’s my favourite constellation because of its distinctive shape and because it appears to have everything. It doesn’t take much to visualise a hunter from the stars in Orion and what stars they are! Orion’s right shoulder is represented by the star Betelgeuse, a variable red supergiant, varying in magnitude from about 0.3 to 1.2 and the 7th brightest star in the northern hemisphere. If Betelgeuse were to replace our sun it would reach out all the way to the orbit of Jupiter. It also has the potential of going supernova but of course we do not know exactly when. Then, representing his left foot, is the blue supergiant Rigel the 5th brightest star in the northern hemisphere with a magnitude of 0.2. Between these stars is a line of three stars going from south east to north west and they represent Orion’s belt and at magnitudes of around 2 they are unmistakable. Less bright but still visible to the unaided eye is Orion’s sword hanging from his belt. The bottom star of the sword should be visible in good conditions and above this is a misty fuzzy patch which is the Orion nebula (aka M42) where star formation takes place. Try to observe it through binoculars or a telescope if you get the chance.
The rest of this blog is about stars and there will be more to say about the constellations in which they lie next month. Because it is so easily recognisable, Orion is a good starting point for finding your way about the night sky during the winter months and especially for the stars we discussed last month. Follow a line from Orion’s belt to the upper right, underneath the star Bellatrix representing his left shoulder, and you will find the star Aldebaran, a giant red star of magnitude 1 and the 9th brightest star in the northern hemisphere. Continue the line beyond Aldebaran and you find the star cluster- The Pleiades or Seven Sisters.
Having followed the line to the Pleiades turn ninety degrees to the north and the bright star you see is Capella, in the constellation Auriga- The Charioteer, lying directly above Taurus. It is the 4th brightest star visible in the northern hemisphere and shines at magnitude 0.1.
Now follow a line from Orion’s belt to the south east and you will find the brightest star visible in the night sky, Sirius- the Dog Star. At magnitude -1.4 it is twenty three times more luminous than the Sun and a mere 8.6 light years distant. Sirius is part of the asterism known as the Winter Triangle, formed in conjunction with Betelgeuse and Procyon which lies due east of Betelgeuse. The white star, Procyon, is the 6th brightest star visible from the northern hemisphere so it is little wonder that the Winter Triangle is something to behold. It is outlined in yellow in the chart.
We finish this month with another asterism- the Winter Hexagon. It is roughly centred on Betelgeuse and comprises the stars Procyon, Sirius (both in the Winter Triangle), then continuing anti-clockwise, Rigel, Aldebaran, Capella and Pollux. It is outlined in red on the chart. You can enjoy viewing all these throughout the coming winter months as they make their way westwards in the evening sky.
Something to look out for
A solar eclipse takes place on Saturday 4th December but unfortunately it will not be visible from Europe. Look out for coverage in the media because it is always good to see.
Having witnessed the planets aligned last month there is a chance to see each one in turn, Venus, Saturn and Jupiter have a close approach with the Moon on the 7th, 8th and 9th of the month respectively.
Finally the December solstice is on the 21st.
The clocks have gone back again so there should be more opportunities to enjoy the winter sky in the darker evenings. It has been good watching Jupiter and Saturn in the evening sky, especially the former as it shines so brightly. Unfortunately my attempts to see their close approaches to the Moon have been thwarted by clouds. Still there will be another chance this month.
The following chart represents the night sky at 11.00pm GMT on the 8th of November and at 10.00pm GMT on the 23rd November. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart. If you are observing a little earlier in the evening then the view is shifted 15 degrees eastwards for every hour before the specified time.
On the right hand side of the chart there is part of the constellation Andromeda and the two smaller constellations Triangulum and Aries which we mentioned last month. But we will use our old favourite Cassiopeia as our guide because you cannot fail to see it. Remember you don’t have to wait till late in the evening to observe these, make use of the earlier dark skies and look more towards your eastern horizon.
Look to the south east of Cassiopeia and you find, lying in the Milky Way, the constellation Perseus- named after the hero in Greek mythology. Again it is difficult to recognise a human shape but the lines above the star Mirphak represent his right arm and sword while the bright star Algol represents his left hand holding the head of his victim. That is enough of the gory details. Mirphak is a 1.8 magnitude yellow supergiant while Algol is an eclipsing binary varying in magnitude between 2.1 to 3.4. There are another five stars around magnitude three or brighter making Perseus a prominent northern constellation.
To the left of Perseus is the constellation Auriga- The Charioteer. It is easily identified because it contains the star Capella, the 4th brightest star in the northern hemisphere, at magnitude 0.1, and ‘only’ 42 light years from Earth. Again it is a binary system composed of two yellow giants. Auriga is in the rough shape of a pentagon although the bottom star Alnath is actually in the constellation Taurus.
In fact the constellation Taurus- The Bull, is our final constellation this month, lying directly below Auriga. It is one of the oldest constellations having been recognised as early as Babylonian times. It is also one of the constellations of the zodiac. Its brightest star is Aldebaran a red giant of magnitude 1 and readily recognisable because of its colour, supposedly representing the red eye of the angry bull. The two lines with Alnath at the top of the upper one represent the bull’s horns. The ‘V’ shape to the right of Aldebaran represents the face of the bull and is an open star cluster called the Hyades. The more famous open cluster, the Pleiades or Seven Sisters, lies to the north west of Aldebaran in the direction of Algol. The Pleiades is one of my favourite objects to view with the unaided eye and I recall watching Venus pass close to the Pleiades during the spring of 2020.
Something to look out for
Saturn and Jupiter are the gifts that keep on giving as they both make another close approach to the Moon on the 10th and 11th of November respectively. There will also be a partial eclipse of the full Moon on the 19th November but we are not well located for viewing it as the Moon will be close to the horizon and will set partway through the eclipse which takes place between 7.00am and 9.00am.
Since the first discovery of an exoplanet system in 1992, it has become apparent that the structure of the solar Solar system as it exists today is by no means typical of planetary systems in general. The early discovery of hot Jupiters orbiting their stars close in, although a result of observational bias in the radial velocity method, has been confirmed by other detection methods and has led to the concept of planetary migration over timescales of millions to billions of years. Studying the likely history and possible future of the solar Solar system reveals a chaotic environment for the inner planets and a semi-stable environment for the giant planets. Long term orbital instability has profound implications in terms of planetary migration, collisions and ejections for both the solar Solar system and for exoplanet systems.
The evolution of planetary systems has been the subject of study for several hundred years, with Kepler (1571-1630), Newton (1643-1727), Laplace(1749-1827), Lagrange (1736-1813), Gauss (1777-1855) and Poincaré (1854-1912) all having made great contributions to the field. Also, as is well known, the precession of Mercury's orbit was first accurately explained by Einstein's, (1879-1955) theory of General Relatoivity.
Approximately 4.5 billion years ago, gravity pulled a cloud of dust and gas together to form the protoplanetary nebula from which the Sun and the rest of Solar system evolved. Subsequently, by processes of both acretionaccretion and condensation in the gas cloud, planetesimals formed and collisions between them led to the formation of planets. The present Solar system structure is like a flat disk, with objects within the disk orbiting in almost the same plane. The objects generally orbit in the same direction and, with the exceptions of Venus and Uranus, rotate in the same direction.
There are several general types of such objects:
The Sun contains >99% of the mass of the Solar system, whereas the major planets account for
~ 99% of the angular momentum of the Solar system.
Gravitational influence of the giant planets has a major effect on the smallest planets. Jupiter, with the greatest mass of all, perturbs the orbits of the two least massive planets so that Mercury has the most eccentric orbit and Mars the second most eccentric orbit in the Solar system.
Using the 305m Arecibo radio telescope (sadly, now defunct), it was demonstrated that the 6.2ms pulsar PSR1257 +12 is orbited by two or more planet-sized bodies – the first detection of an exoplanet system (Wolszczan & Frail, 1992). Perhaps more well-known, 51 Pegasi is a Sun-like star located in the constellation of Pegasus at a distance of approximately 15 parsecs from the Solar system. 51 Pegasi was the first main-sequence star (type G2IV) found to have an exoplanet - designated by convention as 51 Pegasi b (Mayor & Queloz, D et al, 1995). Until the early 1990s it was widely assumed most planetary systems would be essentially similar to the Solar system. That this is not the case may be explained by considering the whole time-domain of the evolution of all planetary systems. Each system we detect maybe at any stage in its development, and will therefore most likely have different characteristics to the present-day Solar system.
We begin by considering gravitational effects. Generally, a low mass object orbiting a much more massive body would be expected to be forced gravitationally into an almost circular (i.e. zero-eccentricity) orbit. For example, this is the case with the orbits of the Galilean moons of Jupiter, where each moon's mass is much less than Jupiter’s mass (see Appendix: resonant and non-resonant orbits).
Considering just two gravitatonally-bound objects, Newton’s law of gravitation can be written as:
Where the respective forces are as shown in Figure 2.
This is mathematically called a 2-body problem, which as may be seen from the equation has a simple algebraic solution.
We should also recall Kepler's Laws, where he postulated that all orbits are elliptical. In particular, Kepler's Third Law tells us that
where k is a constant of proportionality given by
It turns out that if P is in Earth years and a is in Astronomical Units, (AU), k=1. The orbits of the Solar system’s major planets at the present time conform exactly to Kepler's Third Law as shown in Figure 3 below:
The preceding evidence gives the impression of a well-ordered and static system. We must however remember that what we are seeing at any instant in time is a snapshot in the history of the Solar system. As we shall see, planetary systems are generally much more complicated.
Complexity of multi-body systems
Kepler's Laws are regarded as fundamental Laws of planetary systems, so Kepler's Third Law is assumed to apply throughout the life of the Solar system and that of any exoplanet system. However, this does not mean the configuration over the lifetime of the system is static. The orbits of the system will always be elliptical (and hence conform to Kepler’s Laws), but the parameters eccentricity, major and minor axes of the ellipses will change as the planets interact with each other. The only case where the orbit of a planet will acquire an orbital eccentricity e>1 is in the case of a planetary ejection from the system (Laskar, 1994)
The plot in Figure 4 shows the eccentricities of the major Solar system planets. We can see from this plot that whereas the orbits of Venus, Earth, Jupiter, Saturn, Uranus and Neptune have similar eccentricities, the orbits of Mercury and Mars are clearly outliers. Mercury has a particularly eccentric orbit Ε = 0.2056.
Exactly as one would expect, the Left-to-Right order of the plots is the order outwards from the Sun at which the planets orbit. Of all the major planets in the solar system,. Mars has the second most eccentric orbit with Ε =0.0935.
In Figure 5, we plot orbital eccentricity, Ε against planetary mass, M for each major planet. Here the plots are not in order of distance from the Sun - the rightmost plot is the most massive planet, Jupiter which has therefore the most domiantdominant influence on the rest of the Solar system planets (Hayes, 2010).
Mercury has been described as having the most unstable orbit in the Solar system (Lithwick & Wu, 2014). The root cause of this appears to be that at least the inner Solar system is chaotic, and the outer Solar system is borderline stable (Lithwick & Wu, 2014). What we see here is the situation in the Solar system's time-domain as of today. Beyond a few tens of Myr into the future the motion of the planets based on what we observe today cannot be accurately predicted (Woillez and Bouchet, 2020).
On longer timescales, planetary trajectories can only be studied probabilistically with software using numerical algorithms running on supercomputers (Budrikis, 2020). Simulations using these methods predict that over the Sun’s remaining lifetime around 1% of possible trajectories of the planets show Mercury’s orbit becoming so eccentric it may be involved in a collision with Venus or the Sun (Woillez and Bouchet, 2020). Early studies have discussed the possibility of Mercury being ejected from the Solar system altogether or its collision with Venus (Laskar, 1994). This is also indicated in more recent work which shows the strong influence of the outer planets inducing chaotic variations by the inner planets (Hayes, 2010) . In case these scenaria sound improbable, consider that the most widely accepted theory of the origin of Earth's Moon is that of a proto-Earth colliding with a Mars-size planetesimal.
To better understand this, consider a simple thought-experiment where a planetary system consists of just the Sun, Mercury and Jupiter. The gravitational forces acting on Mercury due to Jupiter and the Sun respectively will be the vector:
The vector components on the right-hand side will be, according to the Inverse Square Law, inversely proportional to the respective vector distances of the Sun and Jupiter respectively, rSUN and rJUPITER:
When Mercury and Jupiter are on the same side of the Sun, these vector components will be at their highest. When Mercury and Jupiter are on opposite sides of the Sun, these vector components will be at their lowest.
As we can see, the two vector sums change continuously as the two planets proceed along their respective orbits, and the calculation of the vectors is relatively tedious..
Now, aside from this thought-experiment, in the real Solar system, we must recall several things, namely:
We can now see that the calculations are of very high complexity, even for a single instant in time, and vastly more complex if we look at the distant past or distant future. This is known as the “N-body problem”, the problem of predicting the individual motions of a group of celestial objects which gravitationally interact. Though analytic solutions have been proven up to N=3 (at least in cases where M1 >> M2) , there is no analytic solution to the N-body problem where N>3 (Heggie, 2005), and instead numerical solutions must be run on computers.
At earlier times in the history of the Solar system, it has been suggested that Jupiter and Saturn were likely in the 3:2 resonance, defined as PSATURN/PJUPITER = 1.5, where PJUPITER and PSATURN are the respective orbital periods of Jupiter and Saturn. Today, this ratio has become 2.49, the resonance having been disrupted by gravitational interactions (Nesvorny, D, 2011)
As planets evolve, their mass is subject to changes. Collisions and absorption of dust small objects increase a planet’s mass, and collisions may either increase or decrease mass. This results in an exchange of angular momentum between an evolving planet and the protoplanetary disc, which causes the planet to migrate through the disc. Until the long-awaited discovery of exoplanets, when planetary formation models were based on the Solar system, planetary migrations were usually considered unlikely.
More recently, data from exoplanet systems has provided strong evidence of planetary migration. For example, WASP-107b, a super-Neptune discovered in 2017, is estimated to have a rocky core of ~ 10 Earth masses, and a large gaseous envelope consisting mainly of H and He. This means WASP-107b’s most probably formed several AU from the host star where the protoplanetary disk is rich in gas, ices and dust (Piaulet et al, 2020). However, the current orbital semi-major axis of WASP-107b is only 0.055±0.001 AU (NASA Exoplanet Archive). This leads to the conclusion that WASP-107b has most likely undergone inwards migration (Piaulet et al, 2020).
Decaying planetary orbits
Have any decaying planetary orbits been observed? Yes, but not that many so far.
The orbital period of exoplanet TrES-1 b, discovered in 2004 (Alonso et al 2004) is getting shorter by around 11 milliseconds per year. This may not sound much, but over an astronomically short time of ~300,000 yr, that's approximately 3 days. The orbital period of TrES-1 b is very close to 3 days according to recognized sources (Exoplanet.eu: P~3.0300722d; NASA Exoplanet Archive: P~3.030070±0.000008d; (see NASA Exoplanet Archive).
Ejection of planets from systems
As mentioned earlier, ejection of small terrestrial planets is plausible (Laskar, 1994). But what about giant planets? Studies of giant planets’ interaction within the protoplanetary gas disk indicate that planetary migration is usual. Moreover, planets emerging from mergers of planetesimals are expected to be in orbital resonance. Planetary systems formed from protoplanetary disks can become dynamically unstable after the gas disappears, since the gas exerts a stabilising influence. This leads to a phase when planets scatter off of each other. According to this model, Jupiter and Saturn were most likely trapped in the 3:2 resonance (Nesvorny, 2011). We can see from Figure 7 that this is certainly not the case today.
Using 6000 scattering simulations, Nesvorny et al evaluated a historic Solar system with both four and five gas giant planets. With four gas giants (i.e. as in the current epoch) the best results were obtained with disk masses between 35 and 50 Earth masses. The fraction of simulations with an initial four outer planets producing a final system also having four outer planets was only between 10% and 13%, showing an unlikelihood that the Solar system evolved from a four giant planet system. When run with a five outer planet system as the initial configuration, simulations showed it roughly 10 times more likely to obtain a Solar system analog.
The conclusion reached by Nesvorny, postulating a "jumping Jupiter" scenario, is that the fifth giant planet was ejected from the early Solar system about 3Myr ago. It should be noted that to explain the current orbital parameters of Jupiter, there is a dependency on the 3:2 resonance between Jupiter and Saturn mentioned earlier. More recently, investigation into the framework of this jumping-Jupiter model assessed the possibility that the high eccentricity and inclination of Mercury originated during the instability and concluded instability can produce the presently large values of eccentricity and inclination of Mercury. (Roig et al, 2021)
How plausible is it that a planet - in the case of the 2011 study by Nesvorny study, a giant planet - could be ejected from a planetary system, becoming a lone planet? An observational detection of a giant-mass lone planet using gravitational micro-lensing has been made (Mroz et al, 2018). In the case of this study, there are some major uncertainties as to where the lens is located. If it is in the Galactic disk, the lone planet should be of Neptune-mass; if the lens belongs to the Galactic bulge population, the lone planet should be a Saturn-mass. It is interesting to note that today’s micro-lensing surveys are able to detect lone planets as small as a single Earth-mass.
The configuration of the Solar system as it is today is atypical of planetary systems we have discovered to date. The current configuration would not have existed throughout the history to date of the Solar system, neither will it remain static in the future. The same reasoning apples to exoplanet systems. Since we are examining exoplanet systems as they exist now may explain why most exoplaenet systems discovered to date show very different characteristics and structure to those of the current Solar system structure.
When we are observing exoplanetary systems, we are observing evolving systems at an instant in time. These systems will also have changed over time and will continue to do so in future. The same reasoning applies in the case of the Solar system. Modeling these changes throughout the expected time-domain is only possible by using numerical methods which require large amounts of computer time.
In planetary systems, events such as collisions, both between orbiting objects with other orbiting objects, and between orbiting objects and the parent star are regarded as normal rather than exceptional. Migration of planets from their current orbits is also not unusual. Planetary ejections also occur, which may be one reason for the existence of some lone planets.
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Mroz, P (2018). A Neptune-mass Free-floating Planet Candidate Discovered by Microlensing Surveys. ApJ , 155:121 (6pp), 2018 March https://iopscience.iop.org/article/10.3847/1538-3881/aaaae9/pdf Accessed Sep 22 2021. Submitted to ApJ
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Piaulet, C et al (2020). WASP-107b’s density is even lower: a case study for the physics of planetary gas envelope accretion and orbital migration. https://arxiv.org/pdf/2011.13444.pdf Accessed Sep-14 2021
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The author gratefully acknowledges two anonymous referees, whose critiques have resulted in improvements to this paper.
Data sources used in this work
NASA Exoplanet Archive
NASA Index of Planetary Fact Sheets
Appendix: resonant and non-resonant orbits
In a system where a low mass satellite is orbiting a much more massive body, we would expect the satellite’s orbit to be forced gravitationally into an almost zero-eccentricity (i.e. very nearly circular) orbit - a process called circularization.
Consider the case of the Galilean moons of Jupiter, where this is indeed the situation. Although there are 79 known Jovian moons (as of September 2021), the Galileans account for 99.007% of the orbiting system’s mass. In this system, each of the four moons’ mass is much less than Jupiter’s mass, and as Table 2 shows, all the respective orbits are of very low eccentricity.
The reason this system’s orbital configuration is not like that of the Solar system is because in the case of the Galilean system, there are no high mass Jovian satellites beyond Callisto, the outermost Galilean moon
A mean-motion orbital resonance occurs when two bodies orbiting a larger primary have periods of revolution that are an in an integer ratio. An example is the resonance between the Galilean moons of Jupiter Io, Europa, and Ganymede, which are in 1:2:4 resonance. That is to say, the furthest orbiting moon, Ganymede makes one orbit, Europa makes two orbits and Io makes four orbits. The fourth Galilean moon Callisto does not cross the zero point simultaneously with the others and is therefore not in orbital resonance with the other three.
Well the autumn equinox is past now and we should begin to enjoy some dark skies again. Remember the clocks go back on the 31st of this month. The Harvest Moon on the 21st September looked quite spectacular as I caught it rising in the east and just fitting between two nearby houses.
The following chart represents the night sky at 11.00pm BST on the 8th of October and at 10.00pm BST on the 23rd October. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart. If you are observing a little earlier in the evening then the view is shifted 15 degrees eastwards for every hour before the specified time.
The chart this month should look familiar because facing south and just above your zenith will be the magnitude 2.3 star ,Caph, a member of five bright stars forming the readily recognisable ‘W’ shaped asterism in the constellation Cassiopeia. Being circumpolar it is visible all year round and six months ago it was low over the northern horizon but now we have a chance to admire it high in the sky.
Below Cassiopeia and to the right is the Great Square of Pegasus which we looked at last month. However, our focus this month is on the constellation Andromeda- the princess and daughter of the mythological Queen Cassiopeia and King Cepheus. Remember that the brightest of the stars in the Great Square of Pegasus, Alpheratz, is actually in the constellation Andromeda so we shall start from there. The main features of Andromeda are two curved strings of relatively faint stars starting at Alpheratz and lying above it and to the left, below Cassiopeia. The bright stars Algol and Mirphak in the constellation Perseus to the east can also help with navigation. The lower string of stars is fairly easily followed by star hopping from Alpheratz: delta Andromeda, Mirach and Almach have magnitudes of 3.3, 2.1 and 2.2 so no problem. The higher string of stars is fainter and poor viewing conditions and light pollution will make them difficult to see with the unaided eye as they are typically of magnitude 3.5 to 4.5. The constellation Andromeda is home to one of the most famous objects in the night sky- the Andromeda galaxy also known as M31 and shown on the chart by a red cross. The Andromeda galaxy is the nearest large galaxy to Earth and is similar in many ways to our Milky Way galaxy and is the only one visible to the unaided eye in the northern hemisphere. To locate it for observing, (your eyesight needs to be pretty good as its magnitude is 3.4), start at Alpheratz and by star hopping, jump to the second pair of stars along the curved strings and extend a line from Mirach through the second star and the Andromeda galaxy will be at a distance approximately equal to the distance between the two stars. Now some mind boggling statistics- the distance to the Andromeda galaxy is about two and a half million light years which means that the light entering your eyes from Andromeda set out two and a half million years ago. Andromeda is the most distant object you can see with the unaided eye but you will need a dark site with no light pollution and clear skies. Don’t expect to see something like the images shown in the gallery of the WMA website, you will have to settle for something which might be described as a smudge or fuzzy star but that doesn’t detract from the sense of achievement. Perhaps we will be able to do it together when we meet at Oakhill on the 9th October. Let’s hope for clear skies.
Only two stars in the constellation Pisces- The Fishes, are brighter than magnitude 4 so it offers little to the unaided eye. The constellation, Triangulum- The Triangle, is equally insignificant but because of its compact size, and a shape matching its name, it is relatively easy to spot.
We’ll finish this month with another zodiacal constellation Aries- The Ram. In Greek mythology it represents the golden ram whose fleece was sought after by Jason and the Argonauts. I can see no resemblance to a ram but its brightest star, Hamal, shines brightly at magnitude 2.0 and can be readily picked out.
Something to look out for
There will be a close approach of the Moon with Saturn and Jupiter on the 14th October and 15th October respectively. Perhaps I should explain that the chart in this blog is selected because it gives the view of the stars shown as they are crossing our meridian and therefore at their highest in the sky and as far away as possible from atmospheric disturbance closer to the horizon. However it is good to take a wider view from time to time and if you look west you can see the Summer Triangle with Altair getting closer to the horizon before it disappears from view later on. Similarly if you look towards your eastern horizon you will catch the beautiful star cluster known as the Pleiades or Seven Sisters. And don’t forget to cross from Cassiopeia through the pole star, Polaris, to The Plough which is currently close to the northern horizon and looking like a plough! Clear skies.
The weather hasn't been particularly favourable recently but at least we can look forward to earlier sunsets and the chance to see some stars in the evening skies again. Let's hope there are clear skies as well.
The following charts represent the night sky at 11.59BST on the 8th of August and at 10.59pm on the 23rd of August. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart.
Having described the Summer Triangle last month we shall use that as our starting point for navigating the skies. Locate Altair at the bottom of the Summer Triangle and continuing on a line from the two small constellations we introduced last month, Sagitta- The Arrow and Delphinius- The Dolphin, we find the smallest constellation in the northern hemisphere night sky Equuleus- The Little Horse or Foal. It is supposed to represent the head of a young horse. Its brightest star, Kitalpha, is only of magnitude 4 and because it is so small this constellation is easily overlooked.
Between Equuleus and the horizon is the zodiacal constellation Capricornus- The Sea Goat. It is relatively small with stars dimmer than magnitude 4 so it doesn't offer much to the unaided eye. Similarly to the left of Capricornus is another zodiacal constellation Aquarius- The Water Carrier and it is equally faint and indistinct. However this month they are the region of the sky in which we find the two planets Jupiter and Saturn of magnitudes -2.8 and 0.2 respectively and these two will outshine any nearby stars and there are more details about them in the final section.
Now let's go to the other end of the Summer Triangle, locate Vega and Deneb and look at some circumpolar constellations.
Facing Deneb, look above your zenith and find Polaris- the Pole star. To avoid neck strain best to use a deckchair! Between Deneb and Polaris is a group of not very bright stars forming a shape similar to the gable end of a house. This is the constellation Cepheus- King Cepheus of Ethiopia in ancient mythology. To the east of Cepheus is the welcome sight of the constellation Cassiopeia- wife of King Cepheus, its familiar 'W' shape now standing on its end. This is a sight you can enjoy more and more in the coming months as it rises higher in the sky until it is at your zenith in the winter months.
To the west of Cepheus is the large constellation Draco- The Dragon, which wraps otself around Ursa Minor. Unfortunately Draco has no stars brighter than magnitude 2 but it does contain the star Thuban which 5,000 years ago was the pole star in the northern hemisphere. Due to the precession of the Earth's axis, the position of the celestial pole traces out a circle, over a period of about 26,000 years, on the celestial sphere and over the last 5.000 years it has moved from Thuban to Polaris. This is rather fortunate for us because at magnitude 2 Polaris is considerably brighter than Thuban at magnitude 3.7.
Finally between Cassiopeia and Cygnus and straddling the Milky Way is the small and obscure constellation Lacerta- The Lizard. Because of its small size it contains few objects of interest but it is the site of 'Nova' explosions, the subject of Hugh's recent talk
Something to look out for
I think we can allow ourselves to get a little excited becuase this month has the potential for some good observation. The planets Saturn and Jupiter are at opposition on the 2nd August and 20th August respectively so they are on the opposite side of the Earth from the Sun and at their closest approach to the Earth. This means that they appear larger and brighter and compared with last year higher in the sky. They rise as the Sun sets and set as the Sun rises so are visible all night being their highest about midnight. Of course they will continue to be visible for some time to come with Saturn having a close approach with the Moon on the 21st August when the 12 day old Moon will pass 7 Moon diameters to the south of Saturn. Similarly there will be a close approach of the Moon and Jupiter on the 22nd August.
Equally exciting is the prospect of an impressive Perseid meteor shower which, although active from 17th July to the 24th August, will peak on the 12th August. What we call a meteor or 'shooting star' is a grain of dust burning up in the Earth's atmosphere. In the case of the Perseids the grains of dust were left by the comet Swift-Tuttle and each year the Earth passes through that dust trail as it orbits the Sun. The meteors appear to radiate from a point in the sky called the 'radiant' in the constellation Perseus, hence the name, but you don't have to be looking at the radiant to observe the meteors. More important is that you can lie back on a lounger, allow your eyes to become accustomed to the dark and avoid light pollution as much as possible. a blanket and warm drink will help to keep you comfortable. If you are watching with a group then a real party atmosphere as I recall when the group had a meeting in Cheddar a few years back and Ben Sutlieff was recording the meteors indoors with his radio equipment. Let's hope for a return to such times soon. Clear skies.
I’m pleased to report that I had some success with observing the partial solar eclipse at the beginning of June. Although there was a bit of cloud cover it cleared up enough to enable me to see somewhere close to the point of greatest eclipse both through solar eclipse glasses and with a homemade pinhole camera. I was with a group of people at the time and they were quite impressed with the result from the latter. There will be another partial solar eclipse next year so for a bit of fun why not try making a pinhole camera. Easy to make with minimum materials.
The summer solstice is past but we still have long light evenings so the following charts represent the night sky at 11.59pm BST on the 8th of July and at 10.59pm BST on the 23rd July. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart.
We shall focus on the jewel of the summer sky this month- The Summer Triangle. It is not a constellation but is an asterism formed by three bright stars from three separate constellations. As usual facing south and looking directly above you just before your zenith you will see a very bright star. It might be easier getting a deckchair out and lying on your back! This star is Vega and you cannot miss it because of its brilliance, it is the 3rd brightest star visible from the northern hemisphere, and it has a grouping of four fairly faint stars in the shape of a parallelogram to its bottom left hand side. Together these stars make up the compact constellation Lyra- The Lyre or Harp. Vega forms a small triangle with the top right hand star of the parallelogram and another star just to its top left. Apparently if you have good eyesight you will observe that this is a double star but closer inspection with a telescope reveals that each of these is a double star. The two pairs of stars are in orbit around each other taking hundreds of years to complete a single orbit and have become known as the celebrated Double Double
The lighter region of the chart to the left of Vega represents the Milky Way, the star filled disc of our galaxy, and there you find a giant cross in the sky and this is the constellation Cygnus- The Swan. The bright star Deneb represents the tail of the san which is flying down the Milky Way. Cygnus is a lovely summer signpost high in the sky. The star Albireo representing the beak of the swan is another double star, beautiful to see through a telescope.
Now from a line joining Deneb and Vega, look down about halfway to the horizon and you find another bright star Altair in the constellation Aquila- The Eagle. Altair is identified by two fainter stars either side of it forming a line pointing to Vega. The three stars Vega, Deneb and Altair form what is called the Summer Triangle depicted in red on the chart. You will enjoy looking at it for the rest of the summer and it is a great help in finding your way around the night sky.
We’ll do that right away to find two small constellations. Contained within the Summer Triangle near the bottom vertex is the constellation Sagitta- The Arrow. It is the third smallest constellation and is somewhat dart shaped but even the brightest star at the point of the arrow is only of magnitude 3.5 so you will need dark sky conditions and a clear sky.
To the left of Sagitta and outside the Summer Triangle is another small but distinctive constellation Delphinus- The Dolphin. The magnitude of its stars is similar to those of Sagitta so again you will need clear, dark sky conditions.
From Altair, looking southerly, drop down the Milky Way and close to the horizon is another zodiacal constellation Sagittarius- The Archer. It is hard to imagine anything that represents an archer but what is easily recognisable is the asterism- The Teapot, outlined in red in the chart. Lying as it does in the direction of the Milky Way, Sagittarius is an area of the sky rich in star clusters and bright nebulae which hide the very centre of our galaxy where there is believed to be a supermassive black hole some 26,000 light years away.
Something to look out for
The planets are not well placed for observing this July so why not enjoy a broad sweep of a starry summer’s evening instead. Start with The Plough and Cassiopeia, which are visible throughout the year, and notice how they have continued on their anti-clockwise journey facing each other across the North Pole with The Plough to your west and Cassiopeia to your east. Now follow the arc of the handle of The Plough down to Arcturus in Bootes as we did in May. Turn slightly east to see The Keystone of the constellation Hercules and continue on to the Summer Triangle discussed above. Take a final look to the east and see if you can pick out The Great Square of Pegasus. Then raise your eyes towards the North Pole and there you see Cassiopeia again. If that doesn’t raise your spirits on a clear summer’s evening I don’t know what will!
Not that long ago, the phenomenon of cryovolcanism was a regarded as a theoretical possibility on some bodies in the outer part of the solar system. On Earth, volcanoes consist of an outpouring of molten rock, mainly silicates. Cryovolcanism is defined as the extrusion of liquids and vapours of materials, mainly water. It was until recently only a matter of scientific speculation and no such thing had been observed.
At the distance at which the gas giant planets orbit the Sun, surface temperatures on planetary moons are very low. In the case of Enceladus, the sixth moon of Saturn (see Figure 1), the warmest surface temperature is around just 70K. At that temperature, water is frozen solid and as hard as granite.
Enceladus was discovered in 1789, one of many discoveries made by the German-born English astronomer William Herschel (1738 – 1822). The moon is very small with a diameter of approximately 500km and little more was known about Enceladus until the late twentieth and early 21st centuries.
The first images of this diminutive moon that showed any surface features were taken by Voyager 1 and Voyager 2. These were not high resolution and taken at long range (so nothing like as detailed as shown in Figure 1).
In 2005, the NASA Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface in much greater detail. We can see that there is very little cratering, suggesting a very young surface. The very high albedo (loosely speaking, reflectivity) of Enceladus’ surface is due to it being composed of water ice. The near-linear surface features located particularly near the poles (Tobie, 2015; Ray et al, 2020) were named “tiger stripes”. The crucial images taken by Cassini revealed water-rich plumes venting from the south polar region as shown in Figure 2 below.
The important point to remember here is that Figure 2 is NOT an artist’s impression. This is an actual observation of active and on-going cryovolcanism taking place right now.
Approximately 93% of the plume material falls back to Enceladus’ The escape velocity at the surface of Enceladus is only ~240 ms-1, so the rest escapes the moons gravitational attraction, and supplies most of the material making up Saturn's E ring. Saturn’s atmosphere does have water clouds, but the water can’t reach as high in the planet's atmosphere as the water from Enceladus showering down. This makes Enceladus the only moon in the solar system known to influence the chemistry of its parent planet.
Evidence from the observed libration of Enceladus plus chemical analysis of the observed plumes using a mass spectrometer instrument aboard the spacecraft provides evidence with a high degree of certainty that the moon has a global sub surface ocean largely (though not entirely) of water (Witze, 2014). Mixed in with water in the plumes are silicates, indicating interaction between the ocean and its underlying rock floor (Choukroun et al, 2021); Tobie, 2015). This is assumed to be caused by hydrothermal venting in the depths of Enceladus’ oceans, which also happens in Earth’s oceans. They are made possible by tectonic processes on Earth, giving the strong implication of similar processes occurring on Enceladus.
A hydrothermal vent known as a black smoker near the Galapagos Islands is shown in Figure 3.
Somewhat astonishingly, around these geothermal vents live primitive life forms generally called extremophiles, since they exists under pressures and temperatures which would kill most species. These extremophiles do not rely on any energy from sunlight and instead receive energy from chemical processes associated with the vent. Given that over 500 such sites are known on Earth, plus the chemical analysis of Enceladus’ plumes, there is a possibility that similar primitive life forms might exist in Enceladus’ sub-surface ocean.
Habitable zones in planetary systems
As far as we know, all living species need water, including extremophiles mentioned in the previous section which may exist on Enceladus. The definition of a stars habitable zone has been the range of distances where liquid water can exist on a planetary surface. Too close to the star, the water will boil off; too far out, the water will freeze. This concept is shown in Figure 4.
This all sounds very simple, but as with many things, the situation is actually more complex. For one thing, the fact that a planet orbits the star within that stars habitable zone does not immediately indicate that the planet is inhabited. Consider for example the case of the Sun. Within the Sun’s habitable zone there are two planetary-sized objects. One, which we call Earth, is full of a vast number of living species. The other, which we call the Moon, is not fare distant from Earth, and within the Suns habitable zone, but is devoid of all life.
The next aspect to consider is the type of star. Red dwarfs are by far the most common type of star in the Milky Way. They have surface temperatures of ~2,000K, and so, referring once again to Figure 4, those stars habitable zones will be closer in to the star than as in the case of the Sun, whose surface temperature is just below 6,000K.
One might reasonably assume that in such a system a planet would be as likely as the Earth to harbour life, albeit the planet would need to be orbiting closer in. For example, the TRAPPIST-1 system in Aquarius, discovered in 2015, has at least 3 earth-sized planets in its habitable zone – see Figure 5.
However, red dwarves are known to produce very intense flares of radiation. For example, studies by NASA’s Chandra X-ray observatory concluded that about 25% of the time, Barnard's Star, located approximately 6 light years from the solar system, unleashes scorching flares, which may damage the atmospheres of planets closely orbiting it. So once again, while the habitable zones around stars such as Barnard’s or TRAPPIST-1 may be long term theoretically conducive to life, in the short term, life may not even get started due to the intense radiation environment.
So let's get back to icy moons. As well as Enceladus, evidence of cryovolcanism has been observed on Neptune's moon Triton and is suspected on Jupiter's moon Europa. And yet, should any of these moons harbour life in their sub-surface oceans, their parent planets - the gas giants Jupiter and Saturn, with the ice-giant Neptune - are well outside the Suns habitable zone. All of these planets would be well off to the right of the diagram in Figure 5.
From the foregoing, we can see that the “conventional” view of the habitable zone can be somewhat misleading. We might assume false positives in the case of red dwarf star systems. Equally we might neglect the cases of life existing in orbits outside of what is conventionally thought of as the habitable zone, such as the possibility of life in the sub-surface ocean of Enceladus.
Choukroun, M et al (2021). Sampling Plume Deposits on Enceladus’ Surface to Explore Ocean Materials and Search for Traces of Life or Biosignatures. The Planetary Science Journal,2:100(7pp), 2021 June. https://iopscience.iop.org/article/10.3847/PSJ/abf2c5/pdf Accessed June 6 2021.
Mackenzie, S et al (2021). The Enceladus Orbilander Mission Concept: Balancing Return and Resources in the Search for Life. The Planetary Science Journal, 2:77(18pp), 2021 April. https://iopscience.iop.org/article/10.3847/PSJ/abe4da/pdf Accessed June 10, 2021
Neveu, M et al (2021). Returning Samples from Enceladus for Life Detection.
Ray, C et al (2020). Oxidation processes diversify the metabolic menu on Enceladus. https://arxiv.org/pdf/2012.08582.pdf Accessed June 11 2020
Tobie, G (2015). Enceladus’ hot springs https://www.nature.com/articles/519162a.pdf Accessed June 10 2021
Witze, a (2014). Icy Enceladus hides a watery ocean. Nature News. https://www.nature.com/articles/nature.2014.14985.pdf Accessed June 10 2021
I don’t know if any of you had more luck than me but I didn’t achieve my own challenge of seeing Mercury during May. With so much rain and cloud cover, and the usual haze close to the horizon observing has been very difficult but I have managed to see Venus on the odd clear night and I still have two evenings left in May to catch Mercury. Several people have mentioned the ‘super’ moon on the 26th so that always attracts attention especially in those parts of the world where a lunar eclipse was observed.
It is the summer solstice in the northern hemisphere on the 21st June so we have long light evenings and need to do our sky observations later than usual and the following charts represent the night sky at 11.59pm BST on the 8th of June and at 10.59pm BST on the 23rd June. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart.
Starting from The Plough like last month we follow the arc of the plough handle down to the bright star Arcturus in the constellation Bootes- The Herdsman. Easily identified by its kite shape. Note that The Plough has continued on its anti-clockwise journey about the Pole star and is now ‘standing on its handle’. Just for interest I’ve named another star in Bootes- Izar, which can be resolved with a small telescope to reveal an orange giant of magnitude 2.7 and a fainter blue star. Something to look out for when we get back to group observing again and the opportunity to make use of a telescope.
Now, while still facing south, look to your left by about 30 degrees (about 3 clenched fists at arm’s length) and you will see a group of four relatively faint stars in the form of a quadrilateral. This is an asterism called The Keystone and is part of the constellation Hercules- the strong man from Greek mythology. Although quite a large constellation, Hercules is relatively faint but The Keystone gives us another signpost in the sky. I can see no likeness to a strong man and in fact Hercules is generally depicted upside down.
Lying halfway between The Keystone and Arcturus is a small but distinctive constellation, Corona Borealis- The Northern Crown. It consists of seven faint stars (but the brightest is of average magnitude 2.2) in a horseshoe shape if you cannot envisage a crown. Corona Borealis forms a very attractive grouping of stars.
The second diagram shows three constellations and the stars are mostly faint but these constellations deserve a mention because they lie on the zodiac, a region of the sky either side of the ecliptic- the apparent path of the Sun as it traverses the celestial sphere. By definition therefore the Sun passes through all the constellations on the zodiac. Historically the zodiac was divided into twelve equal regions each with its own sign of the zodiac and corresponding approximately to twelve zodiacal constellations. These were mostly used by astrologers about which we shall say no more. In the early part of the 20th century the International Astronomical Union defined boundaries for the constellations by coordinates in the sky and irrespective of the star patterns. The zodiacal constellations were no longer of equal size and there were thirteen rather than twelve. The odd one out is Ophiuchus which is a constellation but not a sign of the zodiac. That might be useful as the answer to a quiz question sometime!
Follow a line from the right side of the Keystone down to the horizon and you will see a bright reddish star with an average magnitude of about 1.4. This star is Antares, the brightest star in the constellation Scorpius- The Scorpion. Most of Scorpius and specifically its fish-hook tail is not visible from our latitude. Antares is the 10th brightest star visible from the northern hemisphere and that is because it is a red supergiant and if it were to replace our sun, its surface would lie between the orbits of Mars and Jupiter. It is said to represent the heart of the scorpion.
Having located Antares, the small and faint constellation Libra- The Scales lies towards the right.
Between Antares and the Keystone lies the large but faint and indistinct constellation Ophiuchus- The Serpent Bearer. Ophiuchus does not have much to offer the casual observer but its claim to fame is that it is the home of Barnard’s Star, the fastest moving star in the sky and at a distance of only 6 light years is the fourth closest star to the Sun after the three in the Alpha Centauri system.
Something to look out for
The big attraction in our skies this month is the solar eclipse which takes place on the 10th June. It will appear as an annular eclipse in some parts of the world but as the Moon passes in front of the Sun we will see a partial eclipse starting about 10.00am and finishing just after 12 noon. At maximum coverage at our location the Moon will extend across almost a quarter of the Sun’s diameter between 11.00am and 11.30am.Remember never to look directly at the Sun as this could result in permanent eye damage. Solar eclipse glasses are available if not locally then definitely online. Don’t risk using any unreliable method. Some of you may wish to try projecting an image of the Sun using a pinhole camera. For an image diameter of 1cm you will need a projection tube more than a metre long. Let’s hope the weather stays good.
On the 13th June there will be a close approach of the Moon and Mars with the Moon passing just over 5 Moon diameters to the north of Mars. You will be able to observe this from 10.00pm as dusk fades 16 degrees above the western horizon.
At long last we have been able to enjoy some clear skies and the Moon was prominent high in the sky last month with a close approach to Mars in the middle of April and a super moon on the 27th April.
The following charts represent the night sky at 10.00pm BST on the 8th of May and at 9.00pm BST on the 23rd May. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart.
We’ll start from The Plough which we discussed last month and you will notice that it has continued on its anti-clockwise journey round the pole star and is now slightly west of south with the middle of its handle at your zenith. Follow the arc of the handle of the Plough downwards round to the star, Arcturus, which has the distinction of being the second brightest star visible in the northern hemisphere at magnitude -0.05. Also known as alpha Boo (alpha meaning it is the brightest and Boo a short form of Bootes). It is an orange giant nearing the end of its life and relatively close at a distance of 36 light years. It is the brightest star in the constellation Bootes (The Herdsman) and again it is difficult to distinguish such a figure whereas the Kite asterism is easier to see and is what most people recognise as Bootes.
In mythology the constellation Coma Berenices is supposed to represent the locks of Queen Berenice of Egypt but it contains no stars brighter than magnitude 4 so doesn’t present much to observation with the unaided eye. I recall that it was an answer to a quiz question so if it comes up again at least you will have heard of it.
Carry on following the curve of the arc from Arcturus for about the same distance again until you see another bright star. This is Spica the brightest star in the constellation Virgo- The Maiden. Virgo is of course one of the zodiacal constellations as it lies on the ecliptic. Spica is a blue-white star with an average magnitude of about 1 and is 260 light years from Earth.
It may be easier to memorise these two star hops using the expression (Arc on to Arcturus and Speed on to Spica).
Now look to your north west and from last month you should recognise Leo- The Lion with Regulus shining brightly. Turn to face Leo then look up and you are back at the Plough.
Something to look out for
The challenge this month is to spot the planet Mercury. This is always quite tricky because it is low in the sky and only visible for a short time after sunset in the North West sky. It will be at its brightest early in the month but close to the Sun and it will be at its highest altitude on the 16th May. You will need a clear view to your West/North West horizon and if you have the use of binoculars so much the better. On the 4th May it will be on its easterly journey just below the Pleiades and above and to the left of our old faithful Venus which is significantly brighter. A better opportunity to spot it arises on the 13th May when a crescent Moon, Mercury and Venus form an isosceles triangle about 40 minutes after sunset. I can’t finish without saying how good it is to have Venus back in the evening sky and it will be just above a crescent on the 12th May. Clear skies.