Gliese 581g: Habitable Planet or Statistical Error?
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.
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.
Do you think Gliese 581 g/Zarmina is a real planet?
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.
Sources and Further Information
- The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Sta
S. Vogt, et al. We present 11 years of HIRES precision radial velocities (RV) of the nearby M3V star Gliese 581, combining our data set of 122 precision RVs with an existing published 4.3-year set of 119 HARPS precision RVs.
- Gliese 581g as a scaled-up version of Earth: atmospheric circulation simulations
K. Heng, S. Vogt. We use three-dimensional simulations to study the atmospheric circulation on the first Earth-sized exoplanet discovered in the habitable zone of an M star.
- The HARPS search for southern extra-solar planets XXXII. Only 4 planets in the Gl~581 system
T. Forveille, et al. The Gl 581 planetary system has generated wide interest, because its 4 planets include both the lowest mass planet known around a main sequence star other than the Sun and the first super-Earth planet in the habitable zone ...
- Aliases of the first eccentric harmonic : Is GJ 581g a genuine planet candidate?
G. Anglada-Escudé, et al. The radial velocity (RV) method for detecting extrasolar planets has been the most successful to date.
- NASA - NASA and NSF-Funded Research Finds First Potentially Habitable Exoplanet
A team of planet hunters has announced the discovery of a planet with three times the mass of Earth orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone."