Nearly Earth-sized Planet, Possible Watery World Spotted Near Another Star

Article written: 21 Apr , 2009
Updated: 26 Apr , 2016

Astronomers are announcing a newly discovered exoplanet in the habitable zone of its star, and another one — in the same system — that’s just twice the size of Earth.

The Gliese 581 planetary system now has four known planets, with masses of about 1.9 (planet e, left in the foreground), 16 (planet b, nearest to the star), 5 (planet c, center), and 7 Earth-masses (planet d, with the bluish colour).


This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area, showing that Gliese 581 d is located inside the habitable zone around its low-mass red star. Based on a diagram by Franck Selsis, Univ. of Bordeaux.

Michel Mayor, a well-known exoplanet researcher from the Geneva Observatory, announced the find today. The planet, “e,” in the famous system Gliese 581, is only about twice the mass of our Earth. The team also refined the orbit of the planet Gliese 581 d, first discovered in 2007, placing it well within the habitable zone, where liquid water oceans could exist. 

Both planets were discovered by the so-called “wobble method,” using the HARPS spectrograph attached to the 3.6-meter (11.8-foot) ESO telescope at La Silla, Chile.

The gentle pull of an exoplanet as it orbits the host star introduces a tiny wobble in the star’s motion that can just be detected on Earth with today’s most sophisticated technology. Low-mass red dwarf stars such as Gliese 581 are potentially fruitful hunting grounds for low-mass exoplanets in the habitable zone. Such cool stars are relatively faint and their habitable zones lie close in, where the gravitational tug of any orbiting planet found there would be stronger, making the telltale wobble more pronounced.

Many more exoplanets have been discovered using the transit method being employed by NASA’s Kepler mission: as planets pass between their host stars and Earth, they cause an observable, periodic dimming.

Planet Gliese 581 e orbits its host star – located only 20.5 light-years away in the constellation Libra (“the Scales”) — in just 3.15 days.

“With only 1.9 Earth-masses, it is the least massive exoplanet ever detected and is, very likely, a rocky planet,” says co-author Xavier Bonfils from Grenoble Observatory. Being so close to its host star, the planet e is not in the habitable zone. But another planet in this system appears to be.

“Gliese 581 d is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star,” added team member Stephane Udry. The new observations have revealed that this planet is in the habitable zone, where liquid water could exist. “‘d’ could even be covered by a large and deep ocean — it is the first serious ‘water world’ candidate,” he said.

Mayor said it’s “amazing to see how far we have come since we discovered the first exoplanet around a normal star in 1995 — the one around 51 Pegasi. The mass of Gliese 581 e is 80 times less than that of 51 Pegasi b. This is tremendous progress in just 14 years.”

But the astronomers aren’t finished yet. “With similar observing conditions an Earth-like planet located in the middle of the habitable zone of a red dwarf star could be detectable,” says Bonfils. “The hunt continues.”

The findings were presented this week at the European Week of Astronomy & Space Science, which is taking place at the University of Hertfordshire in the UK. The results have also been submitted for publication in the research journal Astronomy & Astrophysics. A preprint is available here.

Source: ESO. (The site also offers numerous videos about the find.)

18 Responses

  1. Curtis says

    I feel I must point out that nowhere in this article was a test to see if there was in fact water. surely there is some spectrograph data to show the signature for water. The fact that it is in the habitable zone of a host star is exciting, yes, but science is based on imperical observation and data. It is bad science, to say the least, when you start making sensationalist claims.

    Don’t get me wrong, I enjoy your articles, however I would rather hear about the science, not the sensational.

  2. BrianV says

    One of the definitions of “habitable zone” is that liquid water may exist under some circumstances. No one is claiming there is liquid water, only that it may exist under the right circumstances on Gliese 581d.

    It would be fantastic to detect water spectroscopically on this planet. As I understand it, that would require a light source (like a star) to pass behind the planet for a short time. Then astronomers might detect spectroscopic absorption lines of water at the limb of the planet, where the occulted star’s light is absorbed by the atmosphere.

    That would be quite a discovery!

  3. Jon Hanford says

    Great story, Anne, on a planetary system that growing by leaps and bounds. I was curious, though when you state “Many more exoplanets have been discovered using the transit method being employed by NASA’s Kepler mission: as planets pass between their host stars and Earth, they cause an observable, periodic dimming.” I was under the assumption that the spectroscopic wobble method has discovered more exoplanets, with transiting planets being discovered only more recently. Or did you mean to say ‘many more exoplanets will be discovered with Kepler’s transit method? I’m just curious, that’s all.

  4. Rob says

    Curtis Says:
    April 21st, 2009 at 7:08 am
    “I feel I must point out that nowhere in this article was a test to see if there was in fact water”

    Unfortunately it was detected by the wobble method. This means there would be no spectrograph, like there would be in the transit method. We haven’t been able to directly observe it yet.

  5. Aodhhan says

    To get any spectrum data, you first have to be able to see the object without a lot of interference. Since these were detected by watching the star wobble, it may not be possible yet.

  6. Tyler Durden says

    Does anyone know if the constellation Libra is in the field of view of the Kepler satellite?

    Probably not (Kepler only covers about 100,000 stars) but it might pick up a transit of 581d if so.

    It’s designed to find planets as low as 1 Earth mass as far away as 1 AU, so 581d – at 7 Earth masses and 0.2 AU – should be a piece of cake comparatively.

  7. Edwin says

    from what I have read its only lower mass red dwarf stars with a Mass of 20% of the sun (0.20 Sol) that produce massive solar flares.

    Gliese with a Mass of .30 solar masses is above this limit.

    At lower masses and diameters the managenit fields are stronger producing larger flares.

    You could also assume the planets maganatic filed woudl offer protection as well

  8. Nick says

    Kepler’s field-of-view is in Cassiopeia – not Libra.
    Other scopes could presumable pick-up a Gliese transit if the system is oriented in such a fashion.

  9. Nick says

    Oops, I mean Cygnus, not Cassiopeia.

  10. Ap says

    Would living near a red dwarf be beneficial to life that much, even in the habitable zone?
    i remember reading somewere before that red dwarfs produce massive solar flares which can strip atmospheres, more stronger than stars like our star.

  11. Stu says

    I’m always glad when a smaller mass planet like this is discovered; just a personal thing, and no disrespect to their discoverers, but I have a hard time accepting the huge, bloated “hot super Jupiters” found whirling psychotically around their parent stars in just a few days *as* planets, you know? :-/

  12. Elias says

    The title needs a small correction. They discovered planet e, Gliese 581 d was already known, they just didnt think that it was in the habitable zone, and with the addition of Gliese 581e they corrected its possition to the habitable zone

  13. Anthony says

    Just a thought but since when must we find a planet closer to our size that’s habitable? Would it not be more beneficial to find one much larger on the off chance that we go there or try and communicate with what may or may not already be inhabiting it? I mean, bigger means more resources…

  14. Michael P. says


    In our solar system, the only planets heavier than Earth are gas giants. And we don’t know if life could live in / on / around one of those. We do know life can life on a rocky planet (we’re on one), thus the enthusiasm for finding one that’s light. And resource abundance doesn’t really apply to how complex life could get on that planet as much as resource availability…

  15. Layman says

    Anthony- Twice the mass means twice the gravity- If you weigh 150 lbs on Earth- then you would feel like you were carrying 300 lbs around on that planet- Which would get mighty heavy mighty fast.
    All of this makes me wonder- due to gravity- how much mass would prohibit the formation of life- I am sure that there is a limit for human life- Fighter pilots pass out under a certain amount of G’s. Does anyone have an answer or ideas on this question?

  16. chris harding says

    Dear Sir(s):
    More to the point would be if such a planet has a Moon similar to the Earth. The lunar tides were required to set the stage for the transfere of life from the sea to the land; a requirement for the appropriate development along the path to truely intelligent beings !. This could be detected too by a fluctuation in the wobble of the planet around its mother star. Such a signiture to be detected would not require much boost to resolution ?. You could get this by repeatedly studying the existing known wobble and noting the level of error. The fluxtuation in errors of measurement would themselves give it away !.
    Best Regards
    Chris. Harding

  17. Your Junior High Science Teacher says

    To the author of this article:

    Please don’t ever write that something is “twice the size” of something else. If you ever write a phrase like this, you have failed as a science writer (and your junior high science or geometry teacher should have failed you for understanding basic measurements too.) Nobody can guess what you mean–is it twice as large in diameter or surface area or volume? Or, actually, in this case, it turns out it was the *mass* you had to be talking about, because we know *nothing* about the size of the planet as it was detected by a gravitic wobble method, and not by a transit method. This is vague, sloppy, confusing writing.

  18. Another Layman says

    To “Layman”:

    Twice the mass does *not* mean twice the gravitational acceleration at the surface (unless the radius of both bodies is exactly the same!) Even if this planet has the same high density as earth, which is the densest planet in our solar system (and it’s probably less dense,) then its radius will be about 1.24 times larger, and its surface gravity will be about 1.24 times that of earth.

    To your other question, life is not critically dependent on surface gravity, especially if you consider that anything floating in a liquid has essentially zero weight. Just like all of the early life on Earth. And some insects submit themselves to accelerations hundreds of times that of earth’s gravity (e.g. the flea or the click beetle,) so living organisms can be quite robust.

    I’d suggest that you read the classic essay “On Being The Right Size” by J.B.S. Haldane. (Just a Google away.)

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