Will the first ‘super-Earth’ in the habitable zone of its star be found around a red dwarf? An M5-dwarf with both mass and radius about a quarter that of the Sun would have 1/200th Sol’s luminosity. That’s interesting for transit purposes, for a planet in the habitable zone around this star would be close in indeed, some 0.074 AU out, with an orbital period of 14.8 days. Its transit probability would correspondingly be raised by a factor of three compared to the Earth-Sun system.
The result, as laid out by the transit survey called MEarth: Detecting such planets should be possible from the ground. Take a look at the live video of what MEarth is doing. Based at the Fred Lawrence Whipple Observatory on Mt. Hopkins in Arizona, the team works with 1976 nearby red dwarfs, visiting each repeatedly in hopes of snaring an ongoing transit, whose information would then be routed to larger instruments for confirmation. They’re looking at targets spread over the entire celestial northern hemisphere and varying the parameters of each observation to the individual target star. And for this survey, the smaller stars are best:
…the most favourable targets for such a transit survey are, in fact, the smallest stars: although these are intrinsically fainter, the reduced count rates are compensated by having deeper transits, and their faintness increases the number of suitable comparison stars available for a given field-of-view. It is important to recall that for small field-of-view observations of single targets, the noise in the comparison light curve can become an important, or even dominant, contributor to the total noise budget. We therefore further choose to concentrate on the smallest stars…
Small, low luminosity stars with possible planets in a habitable zone close enough to the parent to permit ground-based detection — these are exciting thoughts as we tune up our transit methods and await the launch of Kepler. The small radius of M-dwarfs means that any transiting super-Earth is going to block that much more starlight, throwing a clearer transit signature. We can add in the fact that the small stellar mass coupled with a close-in planet also offers a much clearer radial velocity signature for follow-ups.
Working at infrared wavelengths just longer than visible light, MEarth’s eight robotic telescopes will need a total of two years to complete the survey of its target stars. And there’s this on potential findings:
The design study indicates that a yield of 2.6 habitable zone super-Earths would be predicted if the true occurrence of these planets was 10% around our targets, with larger and closer-in planets being easier to detect. A null result would limit the occurrence of > 2 R⊕ super-Earth planets in the habitable zones of late-M dwarfs to be < 17% at the 99% confidence level, a result that again becomes a stronger limit for closer-in planets.
Remember the key advantages of transits. Measuring the planet’s size by examining the amount of light it subtracts from the star’s light can, when combined with radial velocity data, determine the planet’s true mass and help us work out its density. The James Webb Space Telescope, scheduled for a 2013 launch, might then be able to give us spectral information as starlight filters through a planetary atmosphere. Science News has a good story on MEarth, from which this quote by David Charbonneau (Harvard-Smithsonian Center for Astrophysics), whose team is behind the MEarth project:
“My goal is very much to learn about the robustness of life in different stellar environments. If we find planets in the habitable zones of low-mass stars, and determine that these planets have all the right building blocks for life—for example that they are rocky, are at room temperature and have liquid water—but find no life upon them, that would be a very interesting result indeed.”
There are all kinds of reasons why M-dwarfs might be hostile to life, including the consequences of flare activity on closely orbiting planets (not to mention the nature of super-Earths themselves). But one step at a time, as we first try to determine just how common such worlds are. The paper is Irwin et al., “The MEarth project: searching for transiting habitable super-Earths around nearby M-dwarfs,” appearing in Proceedings of the 253rd IAU Symposium: “Transiting Planets” (May 2008, Cambridge, MA) and available online.