Home > detection, worlds > Gliese 581 c (confirmed!)

Gliese 581 c (confirmed!)

April 24th, 2007

Gl 581 c

Image Source.

Big news today from the Geneva extrasolar planet search team. Using the HARPS instrument at La Silla, they have announced the detection of an Msin(i)=5 Earth Mass planet orbiting the nearby red dwarf Gliese 581. The planet has an orbital period of 12.9 days, which places it squarely within the habitable zone of the parent star.

The planet probably migrated inward to its current location from beyond the “snowline” in GL 581’s protostellar disk, and so its composition likely includes a deep ocean, probably containing more than an Earth’s mass worth of water. Atmospheric water vapor is an excellent greenhouse gas, so the conditions at the planet’s atmosphere-ocean boundary are probably pretty steamy. It’s also possible, however, that the planet formed more or less in-situ. If this is the case, it would be made from iron and silicates and would be fairly dry. It’s unlikely, but not outside the realm of possibility, that this could be a genuinely habitable world. There’s no other exoplanet for which one can make this claim. In short, it’s a landmark detection.

In 2005, the Geneva team announced the detection of a Neptune-mass planet in a 5.366-day orbit around the star, and they published 20 high-precision radial velocities in support of their detection. These radial velocities have been in the systemic backend database since last summer, and so naturally, when today’s detection was announced, I was eager to see the models that our users have submitted for the Gl 581 planetary system.

The six submitted fits with the lowest chi-square for the system — by flanker (fits 1,2), EricFDiaz (fits 3,5), eugenio (fit 4), and bruce01 (fit 6) — all contain both the known 5.366 day planet as well as a planet with properties (Msin(i)~5 Mearth, P~12.2 days) that are a near-match to the newly announced planet. In the following screenshot, I’ve highlighted Gl 581 b in blue and the newly confirmed Gl 581 c in light orange.


Congratulations, Gentlemen. You made the first public-record characterizations of the first potentially habitable planet detected from Earth.

I’ve gone on record a number of times to emphasize that I have no interest whatsoever in priority disputes regarding who discovered what. It’s a forgone conclusion that the Swiss should receive all of the credit for their detection. The F-test false alarm probability for the Gl 581 c signal based on the 20 originally published velocities is ~25%, and there are thousands of planets that have been submitted to the systemic backend that don’t actually exist. Nevertheless, the systemic users can take a genuine pride in knowing that they were among the first on Earth to sense the existence of this extraordinary new world. I can’t resist dusting off Sir John Herschel’s ringing exhortation to the British Association of the Advancement of Science on Sept. 15, 1846, two weeks prior to the discovery of Neptune.

“The past year has given to us the new [minor] planet Astraea; it has done more – it has given us the probable prospect of another […] Its movements have been felt, trembling along the far-reaching line of our analysis with a certainty hardly inferior to ocular demonstration”

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  1. darin
    April 25th, 2007 at 17:43 | #1


    The paper for this system is here:


    and electronic Radial Velocities (for adding to the console) are supposedly available. No known transits either!

    With the rush of reporting the planet, they also left out any analysis about the orbital stability of the system as you change the inclinations to higher masses. Applying the same techniques you and Eugenio have developed for GJ 876 should be interesting. In particular, may I recommend an “inclination slider” for the console (ideally one for each planet, but one for the whole system would be fine) so that people can play with the true masses. I think that the systemic collaboration is an excellent way to find to find the maximum masses for the system that are (1) stable and (2) consistent with the RV measurements thus far. It’s very cool that the fitters picked up on this planet before its announcement. And its very cool that you’ve provided this opportunity and website.


  2. andy
    April 25th, 2007 at 23:49 | #2

    I dunno, from the SIMBAD B and V magnitudes for this star and the formulae here, I get a luminosity of around 1.9% solar, which, when putting in a solar habitable zone of 0.95-1.65 AU, gives a Gliese 581 habitable zone of 0.13-0.23 AU, which puts Gliese 581 c interior to the habitable zone, and Gliese 581 d on the outer edge. In fact, scaling Venus to this luminosity gives a distance of 0.10 AU, which is still exterior to the distance of Gliese 581 c.

    If I take the stellar radius of 0.38 solar radii and an approximate temperature of 3500 K for the stellar class, I get a similar result.

    At a guess, I’d say the 8 Earth-mass planet Gliese 581 d is much more likely to be the habitable world of this system.

  3. tfisher98
    April 26th, 2007 at 02:28 | #3

    Andy — I am also curious about how one goes about calculating the habitable zone, and don’t really know enough to do more than quote the experts. The Gliese 581 star is desribed in the 2005 Bonfils et. al. paper here:
    “Gl 581 (HIP 74995, LHS 394) is an M3 dwarf (Hawley et al. 1997) with a distance to the Sun of 6.3 pc (Ï€ = 159.52 ± 2.27, ESA 1997). Its photometry (V = 10.55 ± 0.01, B − V = 1.60, Mermilliod et al. 1997; K = 5.85 ± 0.03, Leggett 1992) and the parallax together result in absolute magnitudes of MV = 11.56 ± 0.03, and MK = 6.86 ± 0.04. From its absolute V magnitude and the 2.08 V-band bolometric correction of Delfosse et al. (1998a), the luminosity of Gl 581 is 0.013 L_sun.”

    So they have 1.3% instead of the 1.9% luminosity you calculated. Still a simple square-root law and your range of 0.95-1.65 au for the Sol system would give a comparable range of 0.11-0.19 au for the Gl 581 system. But Udry et. al. in their preprint here
    calculate equilibrium temperatures to be much more pleasant: “From the 0.013 L_sun stellar luminosity (Bonfils et al. 2005b), we compute an equilibrium temperature for the planet of −3 C (for a Venus-like albedo of 0.64) to +40 C (for an Earth-like albedo of 0.35).”

    So the question is (1) how do you calculate equilibrium temperatures and (2) why is the simple square root estimation of habitable zone wrong? Hmmm…

  4. andy
    April 26th, 2007 at 09:03 | #4

    tfisher98 – basically, the way they’ve got the temperature is by assuming that the planet absorbs a certain fraction of the starlight, and equate the energy absorbed and the energy emitted by a blackbody in the shape of a sphere of temperature T.

    Formula ends up as

    T^4 = L(1−A)/(16πσ*a^2)

    Where T is temperature, L is luminosity, σ is the Stefan Boltzmann constant, A is the planet’s albedo and a is the distance from the planet to the star.

    Putting in Venus’s albedo, a luminosity of 0.013 times solar and a distance 0.073 AU does indeed give you around −3 degrees C. Putting in the parameters of the real Venus in our solar system gives a chilly −20 degrees C, so it turns out that Gliese 581 c receives more radiation from its star than Venus does from ours!

    That suggests to me that once you incorporate the greenhouse effect of an atmosphere thick enough to support liquid water and not freeze out on the darkside of the planet, you’re going to be looking at very unpleasant conditions on the surface.

  5. plagööri
    April 26th, 2007 at 13:31 | #5

    I’am a noob in astronomy, nevertheless (as a swiss) I’ve the following questions:
    Giving the mass of 5M and 1.5r suggest roughly a average density of about 1.5 to earth density (5,5) which is above ferrum! Does this as any effects on all the thinking, that it is an earthlike planet ?
    Giving the closeness of the orbit the planet acts most likely like a moon and chance that it has a synchronous rotation due to tidal locking are high. What role does this play for a possible atmosphere or water-conditions – or the word habitable ?

  6. teros
    April 26th, 2007 at 14:33 | #6

    A quick calculation:

    They say the distance of the planet to the
    star is 0.07 a.u. If the star were so brilliant
    as the Sun, then the planet would have (due to
    the distance square law) ~200 times more radiation
    than us. But they also say that this star is 50
    times less brilliant thant the Sun. Therefore, the
    planet is receiving only 4 times more radiation
    than us. This is equivalent to a planet in orbit
    around our Sun at 0.5 a.u. (again due to the
    distance square law), that is, more or less like
    Venus, which is OUTSIDE the Habitability Zone.
    So that planet would be too hot for having
    liquid water, unless other effects were taken
    under consideration.

  7. darin
    April 26th, 2007 at 19:19 | #7

    As you’ve all found, the habitable zone isn’t a terribly robust definition. The inconsistencies in temperature that you are getting are due to different albedos, i.e. how much of the incoming sunlight energy is absorbed. Andy’s formula above is the canonical way to calculate the effective temperature of a planet. Notice that albedo comes in as (1-A)^(1/4), so different albedos are important, but not overwhelmingly so. I think a more important difference, as mentioned above, is the greenhouse effect given that many of the most abundant molecules in the galaxy are greenhouse gases (water, CO2, CH4).

    To answer plagoori’s question, you must remember that neither the mass nor the radius of this planet are acutally known. The mass is the “minimum mass” and, since radial velocity cannot determine the angle of the system, the mass could actually be much larger. I would be not surprised if it turned out that these planets are all Jupiter-size. The quoted radius of 1.5 Earth radii is the size that the planet would be if it were terrrestial, which is not known. We actually have no idea what the density of Gliese 581c is. This is why the transit technique is so useful… you determine the radius and you know the orientation of the system, so you also know the mass. These give you a density and then you can say that it is either a 5 Earth-mass puffball of Hydrogen or a 5 Earth-mass terrestrial planet likely to contain liquid water.

    In any case, the planet should be synchronized or spin-locked to the star. If there is a substantial atmosphere, it is thought that heat redistribution would be pretty good and the dark-side of the planet relatively warm. Laughlin and Fortney are a few of the people working on this question with more detailed models, so pay attention to the site for more information.

  8. April 26th, 2007 at 21:28 | #8

    Thank you, Darin. That was a very articulate and illuminating explanation of the question at hand regarding Gliese 581c. I look forward to seeing what Greg, Fortney and the others working on this question come up with in the future.


  9. plagööri
    April 27th, 2007 at 07:42 | #9

    Hello all,

    Thank you for all these interesting information and the simualation movie.

    Obviously the mind has shifted slightly because the word “habitable” vanished from the title of the (original) document of April 4th: see here http://obswww.unige.ch/~udry/udry_preprint.pdf

  10. andy
    April 27th, 2007 at 21:37 | #10

    Extrasolar Planets Encyclopaedia now lists the planet having eccentricity 0.16 for planet c – what would that do in terms of pseudosynchronisation/capture into higher order spin states?

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