We’ve been talking for the last six years (since Centauri Dreams‘ inception) about finding a terrestrial world in the habitable zone of another star. It’s an exciting prospect, but the reality about space missions like Terrestrial Planet Finder and Darwin, each designed to make such identifications, is that the budget ax has fallen and we don’t know when they might fly. Indeed, we still face a host of technological difficulties that call for much work if the aim is not only to find a terrestrial world but also to study its atmosphere for possible biomarkers.
Alternatives are therefore welcome, and one is to look for terrestrial worlds around nearby red dwarf stars using transit methods. Usefully, an Earth-size planet orbiting such an M dwarf would be easier to spot than the same size planet orbiting a star like the Sun, and we could use ‘eclipse spectroscopy’ with the James Webb Space Telescope to study such a planet’s atmosphere. Right now we’re making Doppler surveys of nearby M dwarfs, and to good effect, with discoveries like the ‘hot Neptunes’ GJ 436b and GJ 581b, and ‘super-Earths’ like GJ 876d. We’ve also found two planets near to their star’s habitable zone in GJ 581c and d. We should be finding habitable ‘super-Earths’ in the near future with these methods and some of these, let’s hope, will be transiting.
Surveys monitoring thousands of stars can pick up transiting planets (think of Kepler), and Michaël Gillon and colleagues explain in a new paper that most known transiting planets have been detected by such dedicated photometric surveys. The MEarth Project at Mt. Hopkins, AZ monitors nearby M dwarfs with small telescopes and is sensitive to transiting worlds down to a few Earth radii. Gillon’s team is interested in a third approach, one that’s based on a helpful principle. Because planets form within disks, a planet orbiting in the habitable zone of a star will be more likely to transit as seen from Earth if that star already harbors a known transiting planet. From the paper:
Depending on the orbital inclination of the known transiting planet, on the assumed distribution of the orbital inclinations of the planetary system, on the size of the star, and on its physical distance to its HZ, significantly enhanced transit probability can be expected for habitable planets. A dedicated high-precision photometric monitoring of M dwarfs known to harbor close-in transiting planets could thus be an efficient way to detect transiting habitable planets in the near future.
The fact that planets in a system should share similar orbital inclinations is especially useful for M dwarfs because their habitable zones are close to the star. As we discover more transiting planets around M dwarfs (which are currently thought to be the most common class of star in the galaxy), we may be able to use these facts to improve the likelihood of finding habitable worlds. The researchers go on to discuss the potential of this approach for two M dwarfs known to host a transiting planet, GJ 436 and GJ 1214, using a series of simulations.
It turns out that GJ 436 is not a good target compared to GJ 1214. The transit probability of planets in the habitable zone of the latter is much larger. Moreover, GJ 1214 is smaller in radius, meaning that smaller planets could be detected around it. The latter fact also makes for a smaller habitable zone, so that any planet in that zone will be orbiting closer to the star. Ground based monitoring of GJ 1214 could theoretically find a habitable planet as small as the Earth, while space-based observatories like Spitzer could spot a transiting habitable planet down to Mars size.
The planet we already know about here, GJ 1214b, is a super-Earth about 6.6 times the mass of Earth, with a radius somewhat less than three times our planet’s, and it orbits its star every 1.6 days. Roughly 40 light years from the Sun, this system would seem to be ideal for pushing the search for a smaller companion world. The team finds that probing the habitable zone of GJ 1214 would require three weeks of constant monitoring, whereas GJ 436 would require a full two months. That three week run would allow for two transits and could lead to the detection of smaller planets than we’ve hitherto found. The paper confirms the viability of transit surveys like MEarth and offers what may be the shortest course to detecting habitable planets as small, or even smaller, than the Earth. The authors continue:
…we advocate the development of the approach used by MEarth (other facilities spread in longitude, a similar survey observing from the Southern hemisphere, larger telescopes and IR cameras to monitor cooler M dwarfs), but also an intense and high-precision photometric monitoring of GJ 1214 and of the other transiting systems that MEarth (or similar projects) will detect. This two-step approach targeting nearby M dwarfs makes possible the detection in the near-future of transiting habitable planets much smaller than our Earth that would be out of reach for existing Doppler and transit surveys.
The paper is Gillon et al., “Educated search for transiting habitable planets: Targeting M dwarfs with known transiting planets,” submitted to Astronomy & Astrophysics (preprint available). The betting here is what it has always been, that our first detection of a terrestrial exoplanet that is unequivocally in the habitable zone of its star will be around an M dwarf. We’re likely to spot a growing number of habitable ‘super-Earths’ in coming years, so methods that will allow us to extend our discoveries to Earth-size planets are all to the good. After all, who knows how long it will be until funds become available for the kind of terrestrial planet hunter mission we’ve long wished for?