A Neptune-class planet has been discovered around the nearby red dwarf GJ 674, and it’s an intriguing one. Using the HARPS spectrograph on the European Southern Observatory’s 3.6 meter telescope at La Silla (Chile), the discovery team determined that the new planet was 0.039 AU from its parent star, yielding a temperature of some 450 degrees K. With a minimum mass estimate of about 11 times the mass of Earth, it completes an orbit every 4.69 days. Whether GJ 674 b is largely gaseous or rocky is unknown, although further observations of its orbital eccentricity may yield clues.
We’re not down to Earth-mass planets yet, but this is an interesting find. This is the second-closest known planetary system (after Epsilon Eridani). GJ 674 is less than 15 light years away and it’s one of the brightest M dwarfs in our field of view. That makes the transit situation interesting, as Greg Laughlin noted in this systemic post:
At first glance, such an effort might appear to be hampered by the fact that the star is young enough to show significant photometric variability in synch with its 35-day rotation period. A central transit, however, would have a duration of only ~80 minutes — much shorter than starspot-induced variations — and would generate a clearly detectable dip of at least ~0.5% photometric depth.
Such a transit would also help us nail the composition of the planet, no small benefit. But whether or not the Transitsearch team finds a transit here, the broader story emerging from our study of these small red stars continues to develop. Consider this: Two red dwarfs are known to host giant planets — GJ 876 and GJ 849 — and both are relatively metal rich in the M dwarf scheme of things. M dwarfs with planets appear to be slightly more metal-rich than those without, although the authors of this paper are reluctant to push the data too hard given the relatively small sample.
But we can draw other conclusions with a little more force. Laughlin and his collaborators at the University of California (Santa Cruz) have been arguing for some time that according to the ‘core accretion’ model of planet formation, we should find Neptune-mass planets around M dwarfs but not many Jupiter-class worlds. The discovery team for GJ 674 points out that none of the 300-plus M dwarfs examined for planets using radial-velocity techniques has yielded a hot Jupiter, whereas GJ 674b is the fourth hot Neptune found.
Significant? Here’s what Xavier Bonfils (Observatório Astronómico de Lisboa) and collaborators have to say in their paper on this work:
Though that cannot be established quantitatively yet, these surveys are likely to be almost complete for hot Jupiters, which are easily detected. Hot Neptune detection, on the other hand, is deﬁnitely highly incomplete. Setting aside this incompleteness for now, simple binomial statistics shows that the probability of ﬁnding no and 4 detections in 300 draws of the same function is only 3%. There is a thus 97% probability that hot Neptunes are more frequent than hot Jupiter around M dwarfs. Accounting for this detection bias in more realistic simulations…obviously increases the signiﬁcance of the difference. Planet statistics around M dwarfs therefore favor the theoretical models which, at short periods, predict more Neptune-mass planets than Jupiter-mass planets.
So we’re learning more about how the mass of a star relates to the planets that may form around it. Nor is it insignificant that microlensing surveys that have detected four planets around M dwarfs have revealed two that are apparently below a tenth of Jupiter’s mass, further strengthening the argument that Neptune-mass worlds are likely companions to such stars.
And bear one other thing in mind: M dwarfs are small enough that a given planetary mass exerts a correspondingly greater ‘wobble’ effect on them than on larger stars. “As a result,” says the Bonfils paper, “the detection of an Earth-like planet in the closer habitable zone of an M dwarf is actually within reach of today’s best spectrographs.” But let’s go get that transit first.
The paper is Bonfils et al., “The HARPS search for southern extra-solar planets. X. A m sin i = 11 Mearth planet around the nearby spotted M dwarf GJ 674,” submitted to Astronomy & Astrophysics, abstract available.