The Young Astronomers
Currently viewing the tag: "Exoplanets"
This post was originally produced as an Object of the Day for the Galaxy Zoo forum.
In this post I will be looking at three star systems that share a common theme, they are PSR B1620-26, Kepler-16 and Kepler-47 with the common tie being that all three systems are centred on a binary pair of stars – two stars orbiting one another.[1]
Lets begin by taking a look at PSR B1620-26. The system is located 12400 light years away in the direction of the constellation Scorpius – The Scorpion – within the globular cluster M4. M4 is a reasonably loose association of stars that is around 75 light years across. M4 holds the honour of being the first globular cluster to have any of its component stars resolved into isolated objects. M4 is one of the brightest globular clusters in the sky located just west of the α Scorpii – Antares – taking up roughly as much space in the sky as the full moon.
Credit: Kitt Peak National Observatory 0.9-meter telescope, National Optical Astronomy Observatories; courtesy M. Bolte (Universityof California, Santa Cruz)
The PSR B1620-26 is a highly evolved system containing both a white dwarf and a pulsar (spinning neutron star), indeed the system is estimated to be around 12.2 billion years old (compared to our own solar system which is estimated to be about 4.5 billion years old) potentially making it one of the oldest planet containing star systems in the Milky Way (or perhaps even the universe as a whole).
The planet (PSR B1620-26 b) orbits both stars making it a circumbinary planet. It was first announced in 1993 by a team that was studying the Doppler shifts of the system. At first they thought they were looking at a binary pulsar system (with the white dwarf being identified later) though their results showed that there was a third body within the system. When they calculated this unknown object’s mass they found that it was too small to be a star and thus identified it as a planet – one of the first outside our own solar system to be announced though official confirmation had to wait until 2000 (the first planets detected outside our own solar system orbit PSR B1257+12 – another pulsar).
PSR B1620-26 b is about two and a half times the mass of Jupiter and takes about a hundred years to orbit its parent stars.
The star system as a whole is thought to have had a rather unusual history that you can see documented in this NASA graphic.
Credit: NASA and A. Feild (STScI)
The system’s pulsar is 1.35 solar masses and is rotating at about 100 times a second! The white dwarf is considerably less massive (0.35 solar masses) and the pair of stars orbit each other at an average distance of one AU.
The system faces an uncertain future, it is continuing its approach to the core of M4 and as it does so the density of stars surrounding the system will increase. Why is this so you may be thinking? A common way of thinking about globular clusters is that they are essentially self contained spheres of stars. Whilst this is broadly accurate, the stars are not spread evenly through the sphere. Stars are most densely clustered in the centre and become more widely spaced moving out.
As the surrounding area becomes more and more crowded the chances of a close encounter between two star systems also increases. Within the next billion or so years the system is very likely to have another such encounter with the most likely scenario being that the planet (as it is the least massive body in the system) being ejected into deep space fated to wander the stars alone.
Next lets continue with Kepler-16 (I should here note that if we are to follow the full naming convention, the system should be refereed to as Kepler-16 (AB) to show that we are taking about both stars though that is going to rapidly become tedious for everyone involved I shall keep to the shortened version and you can assume that I am referring to both stars when I don’t identify otherwise). Kepler-16 located 196 light years from Earth in the direction of the constellation Cygnus – The Swan.
The system is centred on two small, dim stars – The primary (16A) is an orange dwarf of spectral class KV. it is just 69% the mass of Sol and only a fraction of the brightness. It counterpart is even smaller at only a fifth the mass of Sol making it a MV class red dwarf.
The pair orbit one another in just 41 days and are separated by just 0.22AU – 22% of the average distance between the Earth and the Sun – which is smaller than Mercury’s orbit which sits between 0.31-0.47AU (the range is due to Mercury’s rather eccentric orbit).
Now to the planet itself Kepler-16 (AB) – b catchy isn’t it ::) so for brevity – 16b
16b is a gas giant a third of the mass of Jupiter and 3/4 its radius. This was the first circumbinary planet detected via the transit method – the reduction in the amount of light coming from the parent star as the planet passes in front of it as observed from Earth.
16b transits both of its systems stars, and they themselves transit each other, I admit that is more than slightly challenging to visualise so here is a visual representation with the two stars in the centre and 16b shown as a small blue\purple dot.
Credit: Silver Spoon (Wikipedia user)
Our final system of the day – Kepler 47
This system has only recently had its planets confirmed by the team working on the Kepler mission and marks their first discovery of a multiple star system with more than one transiting planet.
The system can be found at a distance of 4900 light years from Earth in the direction of Cygnus. Both planets are circumbinary orbiting their parent stars. Both of which are smaller than the Sun with the secondary star just 1% as bright as Sol
The innermost planet (47b) orbits once every 50 days and would thus be much too hot for life as we know it to survive on. The outer planet (47c) orbits once every 303 days and this places it at the outer edge of the systems habitable zone. Life like ours is not expected to have developed on 47c as it is predicted to be a gas giant similar in size to Neptune, though perhaps one of its moons (if it has any!) could be suitable.
47b on the right has three times the radius of the Earth
47c on the left is quite similar to Neptune
Credit: NASA/JPL-Caltech/T. Pyle
The most important aspect of the discovery is that it proves that multiple planet systems can indeed form around binary stars. Under current planetary formation models such systems are very difficult to form and suffer from stability issues throughout their existence. Furthermore, as at least one such planet is within its systems habitable zone it is evidence that such orbital configurations are potentially stable and thus the number of locations for life similar to our own to develop has just been increased!
The Kepler 47 system
Credit: NASA/JPL-Caltech/T. Pyle
[1]For those interested you can read more about binary stars and the various types that exist from my post for the Young Astronomers – Binary Stars Blitzed.
We are becoming accustomed to a steady stream of new planets and solar systems being discovered by one of the many planet hunting projects that are currently in operation. Whilst all are interesting, some are particularly unusual and intriguing. One such system is Kepler 36.
Located 1530 light years away in the direction of the constellation Cygnus – The Swan – Kepler 36 is a yellow subgiant (that is a star somewhere between a main sequence star like our own sun and a true giant like Arcturus) of spectral class G1IV. It is ever so slightly more massive than the sun though has a much larger radius (1.63 times as large to be exact).
What is truly interesting however is not the star itself, but the two planets that have been detected in its orbit.
Kepler 36b (from here on simply 36b) orbits at a distance of less than 11 million miles. 36b is classed as a ‘terrestrial super-earth’ being half as large as the Earth again and having 4.5 times the Earth’s mass. It whizzes round its parent star once every 14 days (for comparison the innermost planet in our solar system Mercury orbits once every 88 days). Its partner Kepler 36c (from here on 36c) orbits at a distance of 12 million miles and has been described as a ’mini-Neptune’. c is roughly 8 times the mass of the Earth but the majority of this mass is spread out over a large region producing a planet who’s average density is just 0.86g/cm3 – less than the density of water.
The two planets are particularly unusual as they orbit their star very close together (this is in relative terms of course!) with their close orbits producing very close conjunctions (occurrences when the location of two or more stellar objects in their orbits brings them very close together) on average once every 97 days. At such a conjunction the two are separated by a just 5 Earth-Moon distances (just over 1.2 million miles).
Such events would produce exceptional sights with the planets dominating the other’s sky. There are physical consequences for such close encounters however, the strong gravitational forces acting between the two would cause significant gravitational tides. Such tides are likely to produce significant geological activity on the rocky 36b, potentially including volcanoes and lava flows. The close pairings have also likely stripped most if not all of 36b’s atmosphere leaving a barren, hot rock inhospitable to any life as we know it.
Kepler 36b and 36c Credit: CfA
For this post we will be looking at the star 55 Cancri
Credit: SDSS
and a slightly less gaudy version after some calibration by your’s truly
Original data credit: SDSS Calibration: Peter Clark
Background
55 Cancri is located reasonably to close to the Earth at just 40.3 light years (+/- 0.3ly) in the direction of the Zodiacal constellation Cancer – The Crab. The system is actually a gravitationally bound, detached binary with the bright primary star – 55 Cancri A – separated from its much dimmer partner – 55 Cancri B – by a distance of 1065 AU (nearly 99 billion miles!).
The primary star is about 95% the mass of Sol and is thus slightly cooler and dimmer (though it can be viewed from Earth by the naked eye with clear very dark skies) whilst the secondary is a cool red dwarf only 13% the mass of Sol and only 0.76% as bright (and is thus only visible through a telescope).
The primary star has nearly double the amount of iron content (186% to be exact) compared to Sol classing it as a rare “super metal-rich” or SMR (with an astronomical metal being any element other than hydrogen and helium).
This abnormally high metal content makes it difficult for astronomers to accurately age the star as the models that would normally be used are more uncertain when dealing with stars of this chemical constitution. As such the estimates range from between 5.5 and 8.7 billion years old depending on what study you choose to look at.
Whilst the star itself is certainly interesting, the bodies in its orbit are worthy on mention in there own right.
There are currently five known exoplanets within the system (all of which orbit the primary star). The first – 55 Cancri Ab – was announced in 1997 with 55 Cancri Ac and 55 Cancri Ad following in 2002. 55 Cancri Ae was announced after the detection of its transits across the disk of its parent star and was revealed to the public in 2004 making the 55 Cancri the first known to have 4 planets (other than our own of course!). The completion of this planetary set came in 2007 with the discovery of 55 Cancri Af adding another milestone as the system was the first detected to contain 5 planets.
The two most interesting of this collection are e & f (I think we have reached the stage where repeating 55 Cancri Ax ad nauseum has become unnecessary) and we shall have a look at each in turn.
Rather than going with alphabetical order I will first deal with f (as the most recent and interesting news centres around new observations of e and I will save the best to last!)
55 Cancri Af
f was the first planet to be detected that spends the entirety of its orbit within its system’s ‘habitable’ zone – read as the region of space surrounding a star where ambient temperatures could allow for liquid water to exist on the surface of a planet – unfortunately the planet itself is unlikely to harbour life as we know it. With a minimum mass of about 0.144 times the mass of Jupiter – half the mass of Saturn – (with the actual figure likely to be 25% larger at 0.18 Jupiter masses), it is most likely a gas giant and as such would have no solid surface on which liquid water could collect or for life to evolve. If the planet has one or more large, atmosphere swathed moons (current technology cannot say one way or the other about their existence) life may have a suitable location to develop.
f orbits the primary star once every 261 days and you can see its orbit displayed on this NASA graphic relative to our solar system
Credit: NASA
55 Cancri Ae
e is the closest planet to the primary star and has an orbital period of just 18 hours. It is just under 7 times the mass of the Earth and is likely to be a terrestrial ‘Super-Earth’ – a rocky world more massive than Earth but less massive than Neptune – though it is a very different beast than Earth.
The new observations from NASA’s Spitzer Space Telescope have been collected by analysing the infra-red light given off by the planet itself – rather than observing how the light from the parent star was altered as the planet track across its disk as seen from Earth.
The observations expose a very interesting world indeed. As could be expected from a planet that orbits just 0.015 AU from its star e is rather warm. Indeed its sunward side is likely to reach 2000 degrees Kelvin – hot enough to melt iron and titanium. The observations also show that the planet is most likely dark in colour whilst also supporting previous ideas about the planet’s structure.
All current observations suggest the planet has a large rocky core surrounded by a layer of supercritical water – the best way of describing this is that the water is under so much pressure and is so warm it can’t make up its mind if it wants to behave as a liquid or a gas – covered over by a blanket of conventional steam.
One of the lead astronomers working on the observations – Michaël Gillon of Université de Liège in Belgium – had this to say about the planet,
It could be very similar to Neptune, if you pulled Neptune in toward our sun and watched its atmosphere boil away
I will conclude with this artist’s impression of the planet beside its parent star
Credit: NASA/JPL-Caltech
You can read more about the latest observations here
This post was originally produced as an Object of the Day for the Galaxy Zoo Forum
Back in late September 2010, Astronomers made an important discovery that may eventually change the way we view our place in the cosmos.
The story begins with the all together average star Gliese 581 The star is a spectral class M3 main sequence dwarf, not dissimilar to the image on the left.
An artist's impression of an M class dwarf Credit: NASA/Walt Feimer
At roughly 31% the mass of the sun but only 1.3% its luminosity (that is taking all wavelengths into account not just visual i.e. its Bolometric luminosity).
The star is classified as a variable star due to fluctuations in its brightness over time.
To be more specific it is a BY Draconis variable star as the variability (if confirmed as it currently falls close to the accepted margin for false detection) is expected to be caused by the presence of starspots (the general name for a sunspot that occurs on a star other than the sun) combined with the star’s rotation.
Whilst the star is itself an interesting object, it is what is orbiting it that has caused the media interest beginning in spring 2007.The star has been a target in the search for Exoplanets for some time, the excitement spread when its second planet – Gliese 581c – was revealed to sit just at the inner edge of the systems ‘habitable’ zone.
The term ‘habitable zone’ for a start deeply aggravates me as it means only that the area is neither too hot or too cold, I would prefer it to be known as the ‘temperate’ zone but perhaps that is just me (18 going on a grumpy 60 year old).
Gliese 581 Credit: Digital Sky Survey / ESO
The word habitable implies that the world is suitable for life in every way not just one. Professional astronomers refer to this zone as the ‘Goldilocks zone’ instead as this only refers to the distance from a star that liquid water could theoretically exist on the surface of a terrestrial (rocky) world. Without leading to the assumption that as planet x is y km from its star then it must have life. All this position means is that planet x is in the most likely area of its system for a planet to have liquid water.
The attention soon waned as it became clear that the planet is likely to have a runaway greenhouse effect creating a Venusian world far too hot for life.
The media hype began again with the discovery of the Gliese 581d which sits at the very edge or just outside the Goldilocks zone and so can be expected to be similar to Mars. Then interest peaked again with the discovery of Gliese 581 e which despite sitting very close to the star was the exoplanet with the closest mass to that of the Earth yet discovered with a minimum mass of 1.94 Earth masses.
The current media extravaganza is centred on Gliese 581 g a planet that sits well within the Goldilocks zone, and within the acceptable mass limits for a stable terrestrial planet meaning that it COULD Potentially be suitable for life. However as I have mentioned the issue is far more complex than just having a planet at roughly the right distance from a star.
The Gliese 581 System. Showing each planet's orbital distance in relation to the projected location of the habitable zone and relative to our own Solar System. Credit: ESO derivative work: Henrykus
Yes could be a life bearing world but there is no proof either way quite yet.
Well as the planet is sitting just 20.3 light years away from us, if it indeed harbours life it would go some way to showing just how common life is likely to be within the universe.
However a spanner may have been thrown into the works.
All exoplanet detections must be confirmed, i.e. by the detection of the planet in more than one dataset. So far Gliese 581 g (and Gliese 581 f, announced at the same time though more mundane as it falls well outside the habitable zone and is expected to be similar to Neptune or a super terrestrial planet) has only been detected in one set of measurements; a set of combined data from HIRES spectrometer on the Keck telescope in Hawaii and the HARPS instrument on the ESO’s La Silla Observatory in Chile.Later measurements taken by HARPS failed to detect the planet, so it may not exist at all.
Though as Martin Rees once said, “Lack of evidence is not evidence for absence.” Who knows the planet may yet be confirmed in later measurements though that’s not quite the end of our story.
During the buzz of attention surrounding the discovery of Gliese 581 c, a radio transmission was sent to the system containing messages selected by users of the social networking site Bebo.The transmission was sent in 2008 and will reach the planet in early 2029 with the potential for a reply from any intelligent life on the planet by 2050.
A member of the Galaxy Zoo Forum (Djj) wrote this limerick for my Object of the Day on the same subject,
“Send a message in radio mime
That we’re here and we’re still in our prime;
Gliese five-eighty-one
Might be sombody’s sun —
We could hear back in forty years’ time!”
I will end this post with one of the more indulgent artist’s impressions of Gliese 581 g:
Gliese 581 g Credit: NASA
For our more technically minded readers, you can obtain the original announcement paper for Gliese 581 g here
NASA is considering funding for new space observatory specifically designed to study the atmospheres of Exoplanets in detail not currently possible.
This new mission is FINESSE - Fast Infrared Exoplanet Spectroscopy Survey Explorer – if granted funding would spend at least two years examining the atmospheres of at least 200 of the currently known Exoplanets – to date 716 have been confirmed.
The mission would employ a highly sensitive infra-red spectrometer to determine the compositions of each of the planets atmospheres, with particular emphasis on molecules such as water, carbon dioxide and methane.
The mission will also look at the temperature profile of each planet, looking for any interesting atmospheric phenomena.
The planets selected for the study will include hot-Jupiters, warm-Neptunes and the terrestrial super Earths.
The mission would be part of NASA’s Explorer Program – a low cost project that has a maximum total budget of $200 million.
If selected for funding the spacecraft could be developed rapidly and could be launched as early as 2016.
An Artist's Impression of FINESSE in Orbit Credit: NASAJPL
You can read more here
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