Mo' Better Planets

The Texas HET group has discovered a couple of very interesting new planets which have not received the attention they deserve, yet.

HD155358 is a 0.9 solar mass G0 main sequence star, it is about 130 light years away, it is about 10 billion years old, and it is now known to have two planets.

It is also metal poor.

Two planets, 0.9 and 0.5 Jupiter masses (/sin(i)), orbital radii of 0.6 and 1.2 AU and near circular orbits.
Stellar metallicity is about 1/5th of solar

Paper is in press in ApJ, I'll pass the link (ApJ preprint - subscription) along when I have access.

We know of over 200 extrasolar planets, almost all Jupiter mass or so, most in short period orbits, and most around metal rich stars. Solar metallicity or higher.

Except for the few that are not around metal rich stars, including some of my personal favourite.

For a number of years, there has been a quiet debate over this; sort of - a handful of people have been arguing and everyone else has mostly been ignoring us.
The question is, whether the abundance of planets around metal rich stars is a sign of planet formation being strongly biased towards metal rich stars (which is not implausible given what we know), or whether the observations are selection effect dominated and that metal rich stars only have a lot of high mass short orbital period planets.
A key data point was a Hubble Space Telescope transit survey of tens of thousands of stars in the globular cluster 47 Tuc, none were found, and it is oft cited as evidence for lack of planets in a metal poor system (about 1/3 of solar, higher than HD155338). It is not, it is evidence for lack of short orbital period high mass planets in 47 Tuc, which is interesting, but not the same.

Question has been whether, if we looked harder, the metal poor stars would have comparable abundance of planets, but have systematically lower mass planets in orbits with longer orbital periods.

It is easy to see that you can argue that metal poor planets might have lower mass planets on average, but why they should maybe be in wider orbits is a subtler issue; and papers have been written suggesting that they ought not to be biased towards longer orbital periods.

Arguably, HD155338 is a first data point to the contrary (ignoring the 3-4 previous data points which are rationalised away as being exceptional; not that exceptions prove the rule...)

This system will be subject of some discussion at meetings this summer, and probably a theory paper or three. I have some half-baked ideas, but they need to be shaken out to see if they amount to anything more than idle speculations.

Anyway, this is now one of my favourite systems, and summer has not really started yet, gonna be more news to come.