Last Friday (13th Jan) we hosted a meeting at the Royal Astronomical Society on “Planetary Systems of Evolved Stars”; that is, planets around dying stars like white dwarfs. By “we”, I mean myself, my Leicester colleague Sarah Casewell (@astronomerslc25) and Prof Boris Gaensicke from Warwick. I should add that most of the organisation was done by Sarah, for which Boris and I are most grateful. The meeting was part of the series of monthly “Specialist Discussion Meetings” the RAS holds at its HQ at Burlington House on Piccadilly in London. In fact, we were allocated the lecture theatre in the Geological Society , which is an excellent and historic venue. This is where the infamous Piltdown Man “discovery” was announced in 1912. I hope the centenary of that unfortunate event is just a coincidence…..Anyway, the RAS not only hosted the meeting, providing refreshments in the lovely Geological Soc library, but also kindly allowed us to invite some key speakers, expenses paid, from Europe. I didn’t take a head count, but across the day I guess we had between 30-40 people attending, including a few early arrivals for the monthly RAS ordinary meeting which followed, which gave an opportunity to advertise our field to some who may be unfamiliar with it. We were pretty pleased overall.
So what did we discuss? Basically, what happens to solar systems after their host stars evolve away from the main sequence into first Red Giants and finally, white dwarfs. I first wrote a paper discussing the possibility of finding planets around white dwarfs and how those systems might have evolved back in 2002, and since then we’ve hunted for such planets from the ground with 8m telescopes and from space with HST and Spitzer. In recent years there’s been a lot of activity in the theory community modelling what happens to solar systems and the planets themselves as stars evolve to the white dwarf stage. Since around 2005 there’s also been many more discoveries of dust disks around white dwarfs which are widely accepted to be the remains of asteroids and small planets that have been ripped apart by the white dwarf’s strong gravity. Indeed, these dust disks not only tell us the fraction of white dwarf progenitors (that’s 90% of stars) that form rocky planets, but also what they are made of. Way better than Kepler 🙂
Of course, you may not have heard about this stuff, which is one of the reasons we wanted to have the meeting at the RAS. Indeed, the most disappointing aspect of the day was the non-attendance and interest from many “mainstream” extra-solar planet investigators, but that’s not new. I’ve long been used to feeling that what I’m doing is somehow “left field”, even within my own department. The better news is that view is slowly changing, and at least some UK groups are getting more funding for this field. Sadly, my team at Leicester isn’t one of them, having lost our PDRA support recently. That’s a gripe for another blog really. Partly it can be attributed to the “Mason cuts” in astronomy funding in the UK finally catching up with Leicester, although I have issues with how STFC are currently distributing some of their PDRA awards……
Anyway, I tweeted some of the science highlighted by our speakers during the meeting and collected these with Storify (below). To summarize:
– The survival of planets to the white dwarf stage is a battle between engulfment by the Red Giant, tidal forces, and mass loss from the host star (Eva Villaver, Madrid).
– Terrestrial bodies as large as Pluto seem to be being disrupted and then accreted onto white dwarfs (Boris Gaensicke).
– Anomalous eclipse timing measurements from close white dwarf / red dwarf binaries (including interacting cataclysmic variables) suggests some may be orbited by circumbinary giant planets (Tom Marsh and Madelon Bours, Warwick). Tom urges caution: other effects may mimic planets, but is “70% sure” in the case of NN Ser and 50% sure in 4 or 5 other systems. These things were found before the Kepler circumbinary planets by the way.
– Instabilities set up in two-planet systems at the white dwarf stage can scatter planets and planetesimals into the inner solar system where they can be disrupted to form the observed dust disks (Dmitri Veras, Cambridge).
– At least 3% of hot subdwarf stars have close, brown dwarf companions (Stefan Geier, ESO, Munich) which may have assisted the formation of those objects through common envelope evolution.
– Close brown dwarf companions to white dwarfs can be irradiated and display extraordinary variability at wavelengths from the optical to mid-infrared (Sarah Casewell, Leicester).
– The number of brown dwarf companions to white dwarfs continues to slowly grow and such objects can be used as “benchmarks” to test evolutionary models, since the white dwarf ages are relatively simple to determine (Avril Day-Jones, Santiago, Chile).
– A very widely orbiting 6-9 Jupiter mass companion to a white dwarf has been found in our Spitzer survey data, but it’s difficult to tell whether it is a a bona fide planet or should be regarded as a brown dwarf (the key is formation mechanism). Planets in orbits equivalent to our solar system and other exoplanet systems have not yet been found around white dwarfs (me).
– But there is a habitable zone. And white dwarfs are so small you can find Earth sized and smaller planets around white dwarfs by the transit method……
The full programme from the meeting is here.
Stefan Geier, Eva Villaver and myself enjoying a post-meeting pint or three in Jeffry Barnard and Private Eye’s favourite Soho watering hole, the Coach and Horses