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Transiting ‘Hot Neptune’ Found

Whether or not Gliese 581 c, that intriguing world that may or may not offer temperatures conducive to life, will make a transit of its star is not yet known. But the principle that radial-velocity searches can identify a planet that is subsequently studied via transit received further validation today with the detected transit of a Neptune-class world around GJ 436. This is the smallest and least massive planet ever examined through transit methods, and it bodes well for future such studies of M-class stars.

The new transit comes courtesy of the Swiss team that includes Michel Mayor and Didier Queloz, recently in the news due to their work on Gliese 581 c — do these guys ever get any sleep? As Andy noted in a comment this morning, this ‘hot Neptune’ orbits closer to its star than the innermost planet of Gliese 581. GJ 436 is an M-class red dwarf, a type of star whose small radius makes the detection of such worlds by transit methods easier than would be the case for solar-type stars. And it gives credence to the idea that the Canadian MOST instrument might be able to detect a transit of Gliese 581 c if the planet is properly aligned.

Transits are chancy, and even here, the planetary passage is said to be ‘almost grazing,’ showing a duration half as long as would be expected for a central transit in front of the star. But it’s enough to yield helpful information indeed. GJ 436 b turns out to have a radius comparable to that of Uranus or Neptune. Here is what we know of its composition, from the discovery paper:

The mass and radius that we measure for GJ 436 b indicate that it is mainly composed of water ice. It is an “ice giant” planet like Uranus and Neptune rather than a small-mass gas giant or a very heavy “super-Earth”. It must have formed at a larger orbital distance, beyond the “snow line” where the proto-planetary disc is cool enough for water to condensate, and subsequently migrated inwards to its present orbit.

This planet, which may hold a small hydrogen/helium envelope, will not light up any media switchboards with questions about habitability. Its atmosphere is assumed to be hot, with temperatures ranging from 520 K to 620 K depending upon the albedo value, and the authors note that greenhouse effects may heat it to still higher temperatures.

So while the discovery team believes there are scenarios in which it could be considered an ‘ocean planet,’ GJ 436 b could hardly sport the kind of oceans we’re familiar with here on Earth:

Because of the high surface temperature, this would imply a steam atmosphere above supercritical water rather than an Earth-like situation. As methane and ammonia have very low condensation temperatures, this scenario would imply migration from a wide orbit.

Supercritical water is formed at high temperatures and pressures, showing no real distinction between its liquid and gaseous state. No gently lapping ocean waves in this scenario — in fact, supercritical water is used on Earth as a medium for destroying toxic substances without releasing emissions into the atmosphere. GJ 436 b would be one hostile place indeed, but what promise it holds for future transit studies. For that matter, are there other planets in this very system? Stay tuned.

The paper is Gillon et al., “Detection of transits of the nearby hot Neptune GJ 436 b,” accepted for publication in Astronomy & Astrophysics Letters, with abstract available.

Comments on this entry are closed.

  • Kurt9 May 16, 2007, 13:46

    So, we’ve found ourselves a “waterworld”, eh? Of course, its not anything remotely habitable. But then, I guess Neptune can be called a waterworld as well, since its got a higher percentage of its mass in the form of water than our dear old Earth.

    I would think that waterworlds would be really common out there. Afterall, the most common compounds in the universe will be methane, CO2, ammonia, and water; depending on the relative quantities of C, O, and N.

  • Hans Bausewein May 16, 2007, 16:26

    More here: http://oklo.org/?p=213

    (thanks to universetoday.com)

    I wonder how we know that it must be made of water.
    The density of water, okay, but that leaves a lot of other chemical possibilities.

  • Fraser Cain May 17, 2007, 14:31

    I find it curious that there hasn’t been an official press release on this discovery. Are extrasolar planets becoming so common that people don’t think it’s even worth the time to get a press officer to write it up?

  • Administrator May 17, 2007, 14:37

    It’s a good question, Fraser. I was surprised as well that this one didn’t generate a press release, not to mention more of a media splash. For those wanting more on GJ 436 b, Universe Today‘s story is here:


  • John Hunter May 17, 2007, 23:22
  • Adam May 18, 2007, 5:06

    Hi Paul & Fraser

    The new planet is kind of like Tenebra from Hal Clement’s “Close to Critical”, which I am re-reading – an ocean for an atmosphere. I’m a bit doubtful about the high-pressure ice thing as the phase diagram of Ice VII indicates a LOT of pressure is needed before the melting point goes way up.

    Our local paper reported on GJ 436b so it hardly escaped notice, but it’s not as press worthy as a (near) habitable planet like a fortnight ago.

  • andy May 18, 2007, 6:21

    Can’t really describe this as a “new” planet – it had its time in the limelight back in 2004, when it was one of the first “hot Neptunes” discovered. Systemic has a bit on why the fact it is transiting was missed back then.

  • george scaglione May 18, 2007, 9:10

    yes my friends, in my opinion extra solar planets are already becomming so “common” that people are not getting as excited!?…already! but i also think that when a real possibility exists that we have found “another earth” then that will become a real 3 ring circus!!! film at eleven time! oj trial-like !!shows how far we have come in a manner of speaking.years ago just one extra solar planet would have been the huge news! thanks,george

  • ljk July 26, 2007, 2:06

    Characterization of the hot Neptune GJ 436b with Spitzer and ground-based observations

    Authors: B.-O. Demory (1,7), M. Gillon (1,2), T. Barman (3), X. Bonfils (4), M. Mayor (1), T. Mazeh (5), F. Pont (1), D. Queloz (1), S. Udry (1), F. Bouchy (8), X. Delfosse (6), T. Forveille (6), F. Mallmann (7), F. Pepe (1), C. Perrier (6) ((1) Observatoire de Geneve, Universite de Geneve, Switzerland; (2) Institut d’Astrophysique et de Geophysique, Universite de Liege, Belgium; (3) Lowell Observatory, Flagstaff, AZ, USA; (4) Observatorio Astronomico de Lisboa, Lisboa, Portugal; (5) School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Israel; (6) Laboratoire d’Astrophysique de Grenoble, Universite J. Fourier, Grenoble, France; (7) Observatoire Francois-Xavier Bagnoud – OFXB, Saint-Luc, Switzerland; (8) Institut d’Astrophysique de Paris, Universite Pierre & Marie Curie, Paris, France)

    (Submitted on 25 Jul 2007 (v1), last revised 25 Jul 2007 (this version, v2))

    Abstract: We present Spitzer Space Telescope infrared photometry of a secondary eclipse of the hot Neptune GJ 436b. The observations were obtained using the 8-micron band of the InfraRed Array Camera (IRAC). The data spanning the predicted time of secondary eclipse show a clear flux decrement with the expected shape and duration. The observed eclipse depth of slightly less than 0.5 mmag allows us to estimate a blackbody brightness temperature of Tb = 709 +-17 K at 8 microns. We compare this infrared flux measurement to a model of the planetary thermal emission, and show that this model reproduces properly the observed flux decrement. The timing of the secondary eclipse confirms the non-zero orbital eccentricity of the planet, while also increasing its precision (e = 0.14 +- 0.01). Using additional new spectroscopic and photometric observations allows us to estimate the rotational period of the star and to assess to an high extent the dynamics of the system and the potential presence of another planet.

    Comments: Submitted to A&A on 25.07.2007 – 6 Pages, 6 Figures, corrected missing references

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.3809v2 [astro-ph]

    Submission history

    From: Brice-Olivier Demory [view email]

    [v1] Wed, 25 Jul 2007 19:50:43 GMT (295kb)

    [v2] Wed, 25 Jul 2007 20:31:37 GMT (291kb)


  • andy January 22, 2008, 9:14

    A ~5 M_earth Super-Earth Orbiting GJ 436?: The Power of Near-Grazing Transits

    It may be the reason transits were not originally detected is because GJ 436b has only recently begun transiting, as its orbit is being perturbed by a companion planet. Turns out there appears to be a candidate companion in the radial velocity analysis that might just do the job.

  • ljk January 6, 2009, 0:36

    Transit Timing Observations of the Extrasolar Hot-Neptune Planet GL 436b

    Authors: Guy S. Stringfellow, Jeffrey L. Coughlin, Mercedes López-Morales, Andrew C. Becker, Tom Krajci, Fabio Mezzalira, Eric Agol

    (Submitted on 3 Jan 2009)

    Abstract: Gliese 436 is an M dwarf with a mass of 0.45 Msun and hosts the extrasolar planet GL 436b [3, 6, 7, 2], which is currently the least massive transiting planet with a mass of ~23.17 Mearth [10], and the only planet known to transit an M dwarf. GL 436b represents the first transiting detection of the class of extrasolar planets known as “Hot Neptunes” that have masses within a few times that of Neptune’s mass (~17 Mearth) and orbital semimajor axis <0.1 AU about the host star. Unlike most other known transiting extrasolar planets, GL 436b has a high eccentricity (e~0.16). This brings to light a new parameter space for habitability zones of extrasolar planets with host star masses much smaller than typical stars of roughly a solar mass.

    This unique system is an ideal candidate for orbital perturbation and transit-time variation (TTV) studies to detect smaller, possibly Earth-mass planets in the system. In April 2008 we began a long-term intensive campaign to obtain complete high-precision light curves using the Apache Point Observatory’s 3.5-meter telescope, NMSU’s 1-meter telescope (located at APO), and Sommers Bausch Observatory’s 24″ telescope. These light curves are being analyzed together, along with amateur and other professional astronomer observations. Results of our analysis are discussed.

    Continued measurements over the next few years are needed to determine if additional planets reside in the system, and to study the impact of other manifestations on the light curves, such as star spots and active regions.

    Comments: 4 pages, 3 figures. To appear in “Proceedings of the 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun”, 2009, AIP Conference Proceedings vol. 1094, ed. Eric Stempels

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0901.0343v1 [astro-ph]

    Submission history

    From: Guy Stringfellow [view email]

    [v1] Sat, 3 Jan 2009 23:40:20 GMT (437kb)