Odds and Ends
On the bus home on Friday, I was grinning like a maniac. Goodness knows how much I must have worried the other passengers! I really was atypically cheerful.
You see, I finally had a chance to start to read Borucki et al. 2011 in detail – this being, of course, the one of the Kepler papers that deals with (amongst other things) the candidate-terrestrial exoplanets. Actual data on credible, habitable-zone planet-candidates … this is something I’ve wanted to see for years, and at one point didn’t think I ever would.
In particular, I was looking at Table 6 of Borucki et al. This is the table where the habitable zone candidate-planets are listed.
And there are some interesting things in there. First off, the smallest detected object is KOI 326.01, with an estimated radius of 0.85 Earths. (This makes it a bit smaller than Venus, but rather bigger than Mars.) This object is in a ~13-day orbit around an M dwarf. It presumably has a lower mass than Earth as well and presumably a lower surface gravity.
There are six HZ-objects with radii < 2 Earths (KOIs 1026.01, 854.01, 701.03, 268.01, the aforementioned 326.01 and also 70.03). Of these all but 326 are larger than 1.7 R_Earth – this is at least partly an artifact of the methodology, which makes it easier to detect bigger planets. (You get a bigger dip in the star’s light – a ‘fatter’ planet blocks more of it!) However, it does appear that there are more planets larger than Earth than smaller than Earth. As the paper points out there seems to be some evidence that the size distribution of planets peaks at about 2-3 times the size of Earth, then the numbers start declining again.
As well as small HZ-planets, there are also some very, very big ones. KOI 1375.01 caught my eye in particular – R_p = 17.88 Earths, implying a diameter of ~228,000 Km. By contrast, Jupiter has a diameter of ~142,000 Km. And Jupiter is about as big as planets can get – beyond something like 1.3 Jupiter radii, electron degeneracy pressure starts taking over in the core, and as you add mass to the object, it actually shrinks a bit. (In other words, you enter the brown dwarf regime somewhere around here.)
Now, if KOI 1375.01 was a hot Jupiter, there wouldn’t be a problem – possibly it was bloating up due to the star’s intense heat. However, it orbits in a 321-day period with an estimated temperature of 300 K (or about 27 degrees C). That’s warm, but hardly roasting – in fact it’s not that much warmer than we are. So it can’t be bloated due to irradiation – there’s just not enough heat!
But, its radius would be about right for a mid-M red dwarf. So I suspect KOI 1375.01 will turn out to actually be a small star, not a planet. This might be relatively easy to test – M dwarfs are proportionately a lot brighter in the near-infrared than F, G or K stars, and KOI 1375 appears to be either late-F or early-G type. (It has a temperature of ~6,200 K listed, which would be consistent with that sort of range.) So if the star shows an unusual NIR excess – i.e. it’s unsually-bright in the infrared – then you’ve got some evidence straight away for an M-dwarf companion. Better yet, the system will very probably be a spectroscopic binary. (And this, in addition, is another use for Kepler – binary stars are extremely important as a test of theoretical models.)
Before I sign off, one more intriguing object. KOI 211.01 has an orbital period of 372.1 days around a star with T = 6072 K, leading to an estimated temperature of exactly 0 degrees C*. This is in fact probably the most ‘Earthlike’ combination of temperature, star and orbit on the list. 211.01 itself is not Earthlike – it has a radius of 14.99 R_Earth, so in fact it’s bigger than Jupiter. But, in fact, it’s almost exactly 1.3 times bigger than Jupiter – a brown dwarf, I wonder?
And if a brown dwarf orbiting in a star’s habitable zone had a terrestrial-sized moon – well, that would make me doubly happy. More crazed beaming on the bus would be in order!
*Note that the Earth’s equilibrium temperature is actually about -19 C – the additional 34 degrees comes from the greenhouse effect of our atmosphere.) If we assume an exactly Earthlike greenhouse effect for a satellite of 211.01, then an equilibrium of 0 would imply a surface temperature averaging 34 C – rather on the warm side, and the tropics would be miserable, but the polar/temperate regions would presumably tend toward our Meditarranean.