A New Red Dwarf Planet

by Paul Gilster on April 6, 2007

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.

andy April 6, 2007 at 14:07

A whole bunch of material about planetary habitability, including the environment of M-dwarf planets has been published in Astrobiology and can be downloaded here. Other papers include experimental results of relevance to panspermia, and the spectral signatures of photosynthesis.

Administrator April 6, 2007 at 15:01

Plenty of good stuff in those files that we’ll be examining soon!

andy April 6, 2007 at 15:42

Looking through the M-dwarf+Coronal Mass Ejection papers, it suggests that a habitable planet orbiting an M-dwarf star faces problems with ultraviolet radiation and may have to have a very high CO2:N2 ratio (an atmospheric mix which is more Venus-like than Earth/Titan-like), otherwise the exosphere temperature skyrockets, leading to atmospheric loss. There’s also the problem that these planets are going to have weaker magnetic fields than Earth.

A habitable planet around an M-dwarf might have to be rather more massive than Earth (which would give a stronger magnetic field, and help keep the atmosphere down)… at least formation of “super-Earths” doesn’t look like a problem in M-dwarf systems, but such planets may well be ocean worlds without a chance of land.

Adam April 6, 2007 at 23:28

Hi Andy

The Ocean Planet nature of M-dwarf Super-Earths is still sub judice. Terrans can reach maybe ~ 5 Earth masses before gas capture becomes a big issue and that’s in a G class star’s disk. An M star disk might have a lower gas density – it doesn’t have enough to make a Jupiter in most cases, for example – and so the mass of the “solid” core could be a lot higher. From that other recent paper, reviewed on this blog, which suggests a “snow storm” formation environment around M dwarfs… well icy mantled “Ocean” planets seem almost guaranteed.

But we won’t know for sure until more transits are in the bag. Water planets will be inherently bigger than silicate ones at all masses, so the first M dwarf “Hot Neptune” transit will be VERY informative.

Adam

philw April 7, 2007 at 9:20

Neptune, water planet. I get it!

Adam April 9, 2007 at 4:33

Hi philw

I’ve always thought it an oddly appropriate coincidence.

There’s a 1/9 chance that the Voyage gravity data was wrong and Neptune actually has a density profile compatible with a liquid water ocean, rather than a super-critical fluid abyss.

Christopher L. Bennett April 9, 2007 at 8:23

I hope this isn’t too off-topic, but what are the prospects for habitability on a planet that’s thoroughly covered in ocean dozens of kilometers deep? As I understand it, our ocean life is dependent on nutrients washed or blown out to sea from the land, and on the cycle of carbon and other substances between land and sea. If all the land were buried under many kilometers of water, would that mean the surface of the ocean planet would be basically a “desert,” devoid of significant amounts of nutrients? Could there be enough convection to bring stuff up from the depths (despite the incredible pressures at those depths)?

andy April 10, 2007 at 6:44

Good question there. Another issue to consider is that if you have enough water, you end up with a layer of high-pressure ice below the ocean, which would further isolate the ocean from sources of minerals. You’d presumably have to rely on convection in the ice mantle to transport minerals from the rocky core to the ocean, how effective this would be is unclear.

MirrorStephen April 12, 2007 at 13:55

Really Old Stars Perhaps Ideal for Advanced Civilizations

http://bcast1.imaginova.com/t?r=2&ctl=EA50:4A48D

ljk September 10, 2007 at 0:31

An m sin i = 24 Earth Mass Planetary Companion To The Nearby M Dwarf GJ 176

Authors: Michael Endl, William D. Cochran, Robert A. Wittenmyer, Alan P. Boss

(Submitted on 6 Sep 2007)

Abstract: We report the detection of a planetary companion with a minimum mass of m sin i = 0.0771 M_Jup = 24.5 M_Earth to the nearby (d = 9.4 pc) M2.5V star GJ 176. The star was observed as part of our M dwarf planet search at the Hobby-Eberly Telescope (HET). The detection is based on 5 years of high-precision differential radial velocity (RV) measurements using the High-Resolution-Spectrograph (HRS). The orbital period of the planet is 10.24 d. GJ 176 thus joins the small (but increasing) sample of M dwarfs hosting short-periodic planets with minimum masses in the Neptune-mass range. Low mass planets could be relatively common around M dwarfs and the current detections might represent the tip of a rocky planet population.

Comments: 13 pages preprint, 3 figures, submitted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0709.0944v1 [astro-ph]

Submission history

From: Michael Endl [view email]

[v1] Thu, 6 Sep 2007 21:14:00 GMT (55kb)

http://arxiv.org/abs/0709.0944

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