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.
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.
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.
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 bluepurple dot.
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.
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!
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.