Here at JENAM we have been in the middle of a Symposium on Exoplanets, and Michel Mayor (who started that whole game) gave a plenary review. There are two odd problems with the exo-planets we have found : they are usually too big, and we nearly always find them around stars with high metallicity. Why ?
I was looking forward to the talk by Tristan Guillot, as he was billed to talk about Corot, but they are not ready with summary results yet. (There are some beautiful light curves though – Corot works). Instead he reviewed what has come out of the various transit surveys, and he highlighted the size problem. From radial velocity wobbles you can get the mass; then from the size of the transit-dip, you get the radius. Exoplanets, which should be similar to Jupiter, show a wide range of radii for a given mass. People have suggested explanations – extra heating source, different opacities, etc – but really it is an unsolved puzzle.
Michel Mayor described how workers in this area are steadily pushing down the mass range – we are now finding Neptune-mass exoplanets, not just “Hot Jupiters”. The smallest so far is a “Super Earth” with 5.1 Earth masses. Both microlensing and high-precision radial velocity measurements of M-stars hold out the prospect of Earth mass planets. Meanwhile, Mayor and his chums have for the first time measured a transit for a Neptune-mass planet – GJ346 (Gillon et al, A&A in press – see this web page). They made this measurement at a beautiful small observatory in the Alps – the Observatoire St Luc, achieving millimag precision. The result ? Spot on just like Neptune. So no size problem here .. but its only the first one ! Even lovelier, using Spitzer, they measured an IR anti-transit… because the planet is bright in the mid-IR, you get a dip in the 8 micron flux when it goes behind the star. As they already have the radius, this gives them a temperature – 709K.
But now here comes another puzzle. According to Mayor, the Neptune-mass exoplanets, unlike the Jupiters, are NOT found especially in high metallicity stars. Wuh ? So of course Mayor was asked why in the question session and he said something confident I didn’t follow. So at the coffee break I asked a friendly UK exoplanet-nut, Hugh Jones, and basically he said “beats me – should be the other way round shouldn’t it ?”. Gas giants are mostly H and He, whereas Neptune (we think) has a solid core which forms first out of heavy elements. (Not clear whether this is rocky or icy I think, but I am not an expert..)
So. Me am puzzled.