- Gliese 581g orbits in its star’s “Goldilocks” zone
- Temperature okay for liquid water – not too hot, not too cold
- If confirmed, will be first potentially habitable planet yet found
A team of planet-hunting astronomers, utilising the HIRES spectrometer on the W.M. Keck Observatory’s Keck I Telescope, has announced the discovery of an Earth-sized planet orbiting a nearby red dwarf star.
The new planet, known as Gliese 581g, is at a distance that places it squarely in the middle of the star’s “habitable zone” where liquid water could exist on the planet’s surface.
If confirmed, this would be the most Earth-like exoplanet and the first bona fide potentially habitable one yet discovered.
To astronomers, a “potentially habitable” planet is one that could sustain life—even the simplest of life—and not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.
The discovery by the team, led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington DC, is based on 11 years of observations made at the Keck Observatory atop Mauna Kea mountain on the Big Island of Hawaii.
“Our findings offer a very compelling case for a potentially habitable planet,” said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. “The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.”
“Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution,” added Paul Butler of the Carnegie Institution.
“Our ability to find potentially habitable worlds is now limited only by our telescope time.”
Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team’s new findings are reported in a paper to be published in the Astrophysical Journal.
Planet of perpetual night and day
The astronomers have deduced that the planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness.
One effect of this is to stabilise the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the line between shadow and light (known as the “terminator”), with surface temperatures decreasing toward the dark side and increasing toward the light side.
“Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” Vogt said.
The researchers estimate that the average surface temperature of the planet is between -31 to -12 degrees Celsius. Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side.
If Gliese 581g has a rocky composition similar to the Earth’s, its diameter would be about 1.2 to 1.4 times that of the Earth. The surface gravity would be about the same or slightly higher than Earth’s, so that a person could easily walk upright on the planet, Vogt said.
In fact, the scientists have reported the discovery of not one but two new planets circling Gliese 581. This brings to six the number of known planets around this star, the most yet discovered in a planetary system other than our own solar system.
Like our Solar System, the planets of Gliese 581 have nearly circular orbits. Gliese 581g has a mass 3 to 4 times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and that it has enough gravity to hold on to an atmosphere, according to Vogt.
A difficult discovery
Although the planets themselves can’t be seen, the effect of their gravitational pull on their parent star can be measured. It shows up as a slight movement, or radial velocity change, in the star.
Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to distinguish the effects of the planets and determine their orbits and masses.
“It’s really hard to detect a planet like this,” Vogt said. “Every time we measure the radial velocity, that’s an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters per second to detect this planet.”
To get that many radial velocity measurements (238 in total), Vogt’s team combined their HIRES observations with published data from another group led by the Geneva Observatory (HARPS, the High Accuracy Radial velocity Planetary Search project).
In addition to the radial velocity observations, co-authors Gregory Henry and Michael Williamson of Tennessee State University made precise night-to-night brightness measurements of the star with one of Tennessee State University’s robotic telescopes.
“Our brightness measurements verify that the radial velocity variations are caused by the new orbiting planet and not by any process within the star itself,” Henry said.
How many habitable planets are out there?
Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.
“If these are rare, we shouldn’t have found one so quickly and so nearby,” Vogt said.
“The number of systems with potentially habitable planets is probably on the order of ten or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number. There could be tens of billions of these systems in our galaxy.”
Gliese 581, located 20 light years away from Earth, has a somewhat chequered history of habitable-planet claims. Two previously detected planets in the system lie at the edges of the habitable zone…one on the hot side (planet c) and one on the cold side (planet d).
While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are sceptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.
“It’s the Goldilocks planet,” Vogt said. “That’s a well-worn analogy, but in this case it fits. We had planets on both sides of the habitable zone—one too hot and one too cold—and now we have one in the middle that’s just right.”
Adapted from information issued by W.M. Keck Observatory / ESO / L. Calçada / NASA / ESA / G. Bacon (STScI).
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