The closest we’ve come so far to identifying Earth-like planets around other stars is in the identification of so-called ‘super Earths.’ Calculations designed to model the composition of such planets say that worlds up to about ten Earth masses are rocky rather than gaseous. Some of these, as we have in the case of Gliese 581, have even excited interest in their possible habitability. We’d like to find ways beyond the now conventional radial velocity and transit studies to identify more such worlds.
Now a new planet may have been found around GJ 436, a red dwarf already known to host a Neptune-mass planet in a tight 2.6 day orbit. This is interesting work because of the methods used. Ignasi Ribas (Institut de Ciències de l’Espai, Spain) and team have taken a close look at the known planet and are arguing it is possible to identify a second world, a super-Earth, through the telltale variations in the transit duration of GJ 436b, the already known ‘hot Neptune.’
Giving the game away is the fact that GJ 436b’s orbit, while scorchingly close to the primary, is not perfectly circular. Why should a planet in such an orbit show an eccentricity as high as 0.15? Possible perturbations from other objects in this system have been investigated by others, but Ribas’ team found room to work in the fact that GJ 436b barely crosses the disk of its star as seen from Earth. Tiny changes in the orbital inclination angle can readily be observed, and if that angle is indeed changing, that would explain why this transit has been so hard to confirm — the 2007 transit detection came as a surprise that contradicted earlier results. Say the authors:
Assuming this hypothesis, it is reasonable to explore the possibility of a perturber that could be responsible for both the relatively large eccentricity and the inclination change, while remaining undetected by the radial velocity measurements.
The pieces of the puzzle begin to come together in a second planet for the GJ 436 system. GJ 436c shows a minimum mass of 4.8 Earth masses, with a 0.6 Earth mass play in the numbers. The authors are the first to point out that they do not consider this an ‘extremely solid detection,’ but argue that the case is strong because the existence of GJ 436c explains the inner planet’s orbit. If the finding is borne out, that would make GJ 436c the least massive planet known to orbit a main sequence star.
The radial velocity data on this system is consistent with a planet that matches up with these properties (but see systemic for a more detailed look, and reservations on this), and may indicate still more planets:
Indeed, the system around GJ 436 shows striking resemblances to that around the M-type star Gl 581 (Udry et al. 2007), and thus its planets may experience changes in the orbital elements, perhaps eventually undergoing transits in spite of a previously null result… Our study provides yet another illustration of the variety of exoplanet systems and highlights the potential for complex dynamical histories that imply sizeable variations of the planets’ orbital elements, like the eccentricity, over timescales of decades.
Thus the value of a ‘near-grazing transit’! Note what’s happening here: We’re examining what we know about one planet and using its characteristics to find the signature of a smaller world. Space-based transit studies should be quite useful in working with such tight transits, and that pushes the limit on what we can detect down to even smaller objects, at least in systems as helpful as the one around GJ 436.
The paper is Ribas et al., “A ~5 M_earth Super-Earth Orbiting GJ 436?: The Power of Near-Grazing Transits,” submitted to Astrophysical Journal Letters and available online.