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Gliese 581g: Habitable Planet or Statistical Error?

Updated on September 8, 2012
Gliese 581
Gliese 581 | Source
Artist's conception of Gliese 581 g
Artist's conception of Gliese 581 g | Source

In September 2010, news media were eager to announce an unprecedented discovery: a habitable planet orbiting a nearby star. Astronomers from the Lick-Carnegie Exoplanet Survey had found a planet three times the size of Earth orbiting Gliese 581, a red dwarf 20 light years away in the constellation Libra. Most significantly, this planet was located in the habitable or "Goldilocks" zone - a region around the star that is neither too hot nor too cold to support liquid water.

The discovery of Gliese 581 g was major news not just in science and astronomy circles, but in the mainstream media - including jokes about the exosolar planet in late-night talk show monologues. The idea of a planet capable of supporting life just twenty light-years away was understandably captivating. Lead researcher Steven Vogt even gave the distant world a catchy name: Zarmina, naming the planet after his wife.

Follow-up research on the Gliese 581 system, however, has put a bit of a damper on the discovery by failing to confirm the Lick-Carnegie team's results. What appeared to be a planet may just have been an error in the data analysis. Zarmina may not actually exist.

Hertzsprung-Russell stellar classification diagram. Red dwarves like GL581 are in the lower right.
Hertzsprung-Russell stellar classification diagram. Red dwarves like GL581 are in the lower right. | Source

GL 581: Our Red Dwarf Neighbor

Gliese 581 is an M-class red dwarf star, making it a member of the most common class of stars in our galaxy. The name Gliese comes from the German astronomer Wilhelm Gliese, who in 1957 published a catalog of nearly 1,000 nearby stars. It is a relatively cool star, burning at 3200 Kelvin (5,300 F) - a bit more than half of the temperature of our Sun. Gliese 581 is also smaller and dimmer than our Sun, approximately one-third the size and mass and one-fifth the brightness.

At 20.3 light years away (119 trillion miles, or 192 trillion kilometers), Gliese 581 is a close neighbor in astronomical terms. However, this is far too distant to directly image planets by telescope. Their presence must be inferred using Doppler spectrometry, a technique more commonly known as the "wobble method."

Detecting the Stellar Wobble

The Doppler effect is best illustrated by the sound of a passing motorcycle - rising as it approaches the observer and lowering as it moves away. The motorcycle's motion compresses the wavelength of the sound waves in front of it and lengthens the wavelength behind it, changing the pitch of its engine sound. Light from a moving star behaves in much the same way, shifting toward the blue end of the spectrum as it moves toward the observer and red as it moves away.

When a planet orbits a star, both objects are actually orbiting a common center of gravity or barycenter. Over one of the planet's years, its gravity tugs the star around in a small circle or "wobble." This tug will appear as a cyclical shift in the color of the star's light from red to blue and back. The time it takes the star to make one complete cycle is the orbital period of the planet and the amount of shift is used to estimate the planet's mass. This information, combined with knowledge of the mass and temperature of the star, can determine the size of the planet's orbit and its possible surface temperatures, as well as the likelihood of it having liquid water.

This method has been used to discover about 90% of the extrasolar planets discovered so far. It is most effective at finding large Jupiter-sized planets orbiting close to their stars, since these produce big shifts over a short time period. Earth-sized planets at Earth-like distances have a far smaller tug on their star that takes years to show up in the observation data, and are usually too small to be noticed around a Sun-sized star. Lower mass red dwarves such as Gliese 581, however, are more easily tugged around by smaller planets, and thus good candidates for finding planets closer to Earth size.

Comparison of the orbits of the six possible Gliese 581 planets and our solar system. Planets not drawn to scale.
Comparison of the orbits of the six possible Gliese 581 planets and our solar system. Planets not drawn to scale. | Source

Poll Time!

Do you think Gliese 581 g/Zarmina is a real planet?

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The Mystery of Gl 581 g

The first planet found around Gliese 581 was discovered in 2005. This planet, designated Gl 581 b, is a Neptune-sized giant orbiting very close to the star, completing one year every five Earth days. Next came c in 2007, a planet with five times Earth's mass orbiting every 12 days. Studies also found planet d, a planet half the size of Uranus with an orbit about two months long. Finally, in 2009, the discovery of Gliese 581 e was announced - a small planet only twice the mass of Earth orbiting the star every three days. In a few short Earth years, astronomers had discovered a four-planet system around a nearby star that could easily fit within the orbit of Mercury. Though these finds were exciting, all of the planets discovered were either too close or too far from the star to be habitable.

The nearby star was one of the prime targets of the Lick-Carnegie Exoplanet Survey, partially funded by NASA and the National Science Foundation. Using 240 measurements of the star taken from the Keck Observatory in Hawaii, and the La Silla Observatory in Chile, Steven Vogt's team found evidence of two additional planets around Gliese 581. The first, designated f, was a massive planet seven times the size of Earth in a 445-day orbit. The second, designated g, was far more exciting - its 37-day orbit put it right in the middle of the star's habitable zone, and its low mass of only three times that of Earth made it a promising candidate for habitability.

If life existed on Gliese 581 g, it would be a rather different existence than what we are accustomed to. Given the age of the system, the planet is likely to be tidally-locked to its star, with one side facing the star at all times in the same way that we always see the same face of the Moon. With no day and night cycle, this would mean that half of the planet would be in perpetual day and half in perpetual night, with the heat from the star baking the day side and the permanent shadow freezing the night side. The most likely place for life would be in a ring of perpetual twilight around the planet's terminator line, though wind currents created by the heating patterns could create habitable temperatures much farther into the day and night sides of the planet.

That is, if it actually exists. Follow-up research using additional measurements from the La Silla Observatory did not find evidence for planets f and g. Further analysis of the Lick-Carnegie team's statistical methodology also called their results into question, putting the very existence of Gliese 581 f and g in doubt. The status of the planets is currently unconfirmed, awaiting further research to provide more definitive answers.

To G or Not To G

The story of Gliese 581 g is an excellent example of science at work. Science is often portrayed by deniers and conspiracy theorists as a hegemonic enterprise - a cabal of illuminati dictating the facts from their ivory tower, conspiring to silence competing ideas. This is very far from the truth. Science is a debate, with the winners determined by preponderance of evidence. In many cases, there is overwhelming evidence in support of an idea or theory. In other cases, such as the existence of Gliese 581 g, the jury is still very much out.


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    • scottcgruber profile image

      scottcgruber 5 years ago from USA

      Update for those curious about binary systems: NASA's Kepler mission recently found the first multi-planet system orbiting a binary star. Kepler-47 consists of a sun-like star orbited by an M-class dwarf every 7.4 Earth days.

      There were two planets found orbiting the pair. One is a super-Earth orbiting every 49 days. The second is a Uranus-sized planet orbiting every 303 days, putting it in the system's habitable zone.

    • Marcus Faber profile image

      Marcus Faber 5 years ago from London, UK

      Gliese 581g is back in the news with Vogt's team reaffirming that their evidence is correct, it's been listed as the number 1 candidate for potential habitability by the Planetary Habitability Laboratory.

    • profile image

      Sparkster 5 years ago

      They also now believe there is an underground ocean on Saturn's moon Titan.

    • Marcus Faber profile image

      Marcus Faber 5 years ago from London, UK

      This is a fantastic hub, your closing comments on science are spot on. Since Gliese 581g there have been some more exciting habitable zone candidates, such as Kepler 22-b which I believe has been confirmed.

    • scottcgruber profile image

      scottcgruber 5 years ago from USA

      I think it's possible. It's difficult to say definitively what kinds of systems could harbor life, since we only know of life existing on one planet. The type of life we know needs liquid water to survive, which is most likely in the habitable zone of a star.

      Binary stars present a few challenges to life. A planet orbiting two stars that are close together would experience some major temperature swings as the two suns eclipsed each other. The gravity of the two stars could also cause planets close to the center of the system to be ejected. On the other hand, simulations have demonstrated that a two-star system could be more likely to form planets as the second star would stir up the dust and gas in the protoplanetary disk.

    • sparkster profile image

      Sparkster Publishing 5 years ago from United Kingdom

      I have another question...

      Another hubber stated that binary star systems cannot harbour life but the scientists have been declaring planets of binary star systems as being in the goldilocks zone.

      So, are binary star systems capable of harboring life?

    • somethgblue profile image

      somethgblue 5 years ago from Shelbyville, Tennessee

      thank you.

    • scottcgruber profile image

      scottcgruber 5 years ago from USA

      No. Only about 1/3 of stars are in binary systems. The majority of stars are red dwarves, which rarely have a binary companion.

    • somethgblue profile image

      somethgblue 5 years ago from Shelbyville, Tennessee

      Can you answer a quick question for me . . . Is it true that every solar system mankind has discovered are binary star systems?

    • scottcgruber profile image

      scottcgruber 5 years ago from USA

      Artblack01: very good point, though I think the media are only partly to blame. The hyping often starts with an overzealous press release from a university PR department, or a funding agency like NASA or NSF that's eager to show a return on their investment. Scientists themselves aren't above a bit of hyping as well, eager to extend the influence of their work through citations. It's completely understandable - science is competitive like any other human endeavor.

      The problem, I think, is that not enough people have the critical thinking skills to distinguish the hype from the facts. That breeds the distrust of science that the creationists and other deniers prey upon.

    • artblack01 profile image

      artblack01 5 years ago from New Mexico

      Many of the discoveries made by science, for which are not yet complete studies, like Gliese 581, are often overly hyped or given too much media attention before the studies are even completed. This makes the particular discovery look like a fraud or a hoax or makes science look bad when in actuality it's the media screwing up science. The media often sensationalizes, exaggerates or even lies about many scientific discoveries. Unfortunately the people who suffer are the ignorant masses who eat up everything they see in the news and don't know enough about science to understand what they have been told.

    • scottcgruber profile image

      scottcgruber 5 years ago from USA

      markbennis: Excellent questions!

      The surface gravity of a planet is a function of its density - mass divided by volume. We can calculate volume from the radius, since volume of a sphere is 4/3*?r^3. Saturn, for example, is 95 times the Earth's mass and has a radius nine times the size of ours. However, its density is very low, so its surface gravity is only about 6% more than Earth's. Uranus is another example - it is four times the radius of Earth and 14 times the mass, but you'd actually weigh 12% less on the surface of Uranus, if it had one.

      To figure out the surface gravity of the planets in the Gliese 581 system, we'd need to know the radius, which we can't from the velocity measurements alone. The estimates I've seen for 581 g are 1.1-1.5 times the gravity of Earth, but these are based on educated guesses of the radius.

      The spectrum of Gliese 581 might be a bit closer to the red end of the spectrum, though I'm not sure how it would look to someone standing on a nearby planet. I'll have to do a bit more research...

    • scottcgruber profile image

      scottcgruber 5 years ago from USA

      derbian62: Good point! Though I wouldn't call those people "skeptics" (or sceptics, if you prefer). I call them "deniers." The biggest skeptics in science are scientists themselves.

    • profile image

      markbennis 5 years ago

      Loved this, I am always fascinated with space and the concepts of space exploration, and although at 20.3 light years away? It would be a truly magical moment to have visual images of these possibly habitual planets. If the planet is estimated as being 3x the size of Earth, would that make its gravitational force estimated as 3x as strong as it is on Earth also?

      If so how would that affect us on a physical level, could we adapt to such a pull on our bodies? Also as a red dwarf next to it, what kind of spectrum of light would it be able to omit, I guess something must be lacking if it is not as vibrant as our Star, Sol?

      Now forgive me if you feel I am putting these questions to you, because I am not and don’t expect you to know the answers to them all. It’s just that these discoveries fascinate me and sparks the creative curiosity, if you know what I mean.

      Anyway, great hub and voted up!

    • profile image

      derbian62 5 years ago

      Thank you, Scott, for a really interesting, well-written and thoroughly researched piece. I think most people are fascinated by the idea of life in outer space, whatever form it may take. You're so right about science: it's the search for truth by means of exploration, experiment and discovery. Sceptics dismiss science for the inevitable mistakes (part of the process), rather than praise it for the wonderful advances made. I know that humanity will succeed in travelling to these distant horizons some day. However, we need to take more care of our own amazing planet and each other, so that we survive long enough to develop the means to get out there. I'd be ready and willing to go - what an adventure!