systemic

I’m always impressed by the efficiency with which red dwarfs pack hydrogen, the stuff of flammable zeppelins, into such a small space: Gliese 1214 is more than twice as dense as led. The density of the Sun, on the other hand, is bubblegum by comparison.

Gliese 1214b’s orbital period is a mere 1.58 days. Its 0.014 AU separation from the system barycenter is the smallest yet measured for any planet. Yet because of the high red dwarf density, the star-planet configuration is actually rather spacious. Here’s the system to scale:

It’s interesting to compare this diagram with that of a genuinely close-in planet such as HAT-P-7b, which actually has a somewhat longer 2.2 day orbital period:

At a given period, a red dwarf fills much less of a planetary orbit than does a Sun-like star. If the occurrence rate of planets at a specified period is the same for stars of different masses, then one needs to look at $\sim(M_{\odot}/M_{\rm RD})^{2/3}$ times more red dwarfs than Sun-like stars to find a given number of transits with a particular period.

Gliese 1214b lies at enough stellar radii from Gliese 1214 that its a-priori transit probability was only about 7%. The Mearth survey currently covers only ~2000 stars, and so the fact that the discovery was made so quickly was probably not luck, but rather points to the existence of a very large number of low-mass planets orbiting small stars.

Let’s face it. The big dough goes to chase potentially habitable transiting planets. With this metric, the red dwarfs come out way ahead. If red dwarfs and Sun-like stars have equal occurrence fractions for planets with Earth’s mass and insolation, then a low-mass red dwarf has roughly four times the probability of a Sun-like star of harboring a transiting potentially habitable planet. Twice the temperature means one-sixteenth the area and the square root of sixteen is four. The red dwarfs also present a number of other advantages, see e.g. here, here, and here.

Ryan Montgomery and I have a recent paper out which foreshadows what I think is the inevitability of transit surveys that use the Mearth strategy to target true-Earth analogs the habitable zones of the lowest-mass red dwarf stars. Mearth  is itself very well-positioned to expand in this direction. I also think that a lot of effort will continue to shift toward improved Doppler-velocity capability in the near-infrared (see, e.g. this recent paper by Jacob Bean and collaborators which describes the use of ammonia gas in a glass cell to imprint a forest of fixed reference lines on a K-band stellar spectrum).

A last note: Twelve-Fourteen-b is likely to become a favorite target for small-telescope observers, so I made sure to add it to the Transitsearch.org candidates table. Now that classes are done for the quarter, I’ve been going through the literature and adding or updating one or two planets a day. It’s tedious work, but I’ve noticed some interesting upcoming opportunities, which I’ll be writing about soon. For transit-themed ephemera and the latest celebrity gossip, look no further than the transitsearch twitter stream: http://twitter.com/Transitsearch.

And a postscript: In the comments, reader cwmagee points out that the implication of the post is that the HAT-P-7 and Gl1214 diagrams are to scale which eachother, but that’s not the case. He attached a version which shows a to-scale comparison of both systems:

Red dwarfs are small!

Turn your world upside-down and you’re looking at a very different planet. Antarctica, ringed by the vast exapse of the Southern Ocean, draws all the attention. Viewed from beneath, I think Earth might better resemble the habitable planets that are out there in the local galactic neighborhood, waiting to be found.

Speaking of upside-down planets, last week brought a curious back-to-back development. Three separate papers (one, two, three), posted to astro-ph on two successive days, presented strong Rossiter-McLaughlin-based evidence that both WASP-17b and HAT-P-7b are on severely misaligned, potentially retrograde orbits around their parent stars. Winn et al.’s data for HAT-P-7 are a near-exact inversion of the familiar sawtooth produced by well-behaved hot Jupiters such as HD 209458b or HD 189733b. It would appear that Dr. Kozai exerted a heavy hand during HAT-P-7b’s early days:

The HAT-P-7 system is alarmingly compact. The star is roughly 80% larger than the Sun, and the orbit of the transiting planet is only about four times larger than the star itself. It looks, in fact, when drawn to scale and tilted to the proper inclination, like a schematic cartoon of a transiting system.

Remarkably, HAT-P-7 lies in the Kepler field, and was the subject of a teaser-like “brevia” published in Science a few weeks ago. In the folded Kepler light curve for HAT-P-7b it’s easy to see the phase function of the orbiting planet, along with the primary transit and the secondary eclipse. The well-resolved depth of the secondary eclipse indicates that the spacecraft is performing up to spec and will be able to detect the transits of Earth-sized planets orbiting Sun-sized stars.

Interestingly, a near-perfectly inverted Rossiter-McLaughlin waveform doesn’t necessarily mean that the planetary orbit is retrograde, but rather only that the angle between the planet’s orbital angular momentum vector and the sky-projected spin axis of the star is close to 180 degrees. If the star’s polar axis is pointing nearly in our direction, then the planetary orbit is close to polar. The small vsin(i) for HAT-P-7 provides a piece of evidence that HAT-P-7b’s orbit might in fact be close to polar.