Astronomers have found the first alien world that could support life on its surface. It is both at the right distance from its star to potentially harbour liquid water and probably has a rocky composition like Earth.
“That’s the most exciting exoplanet I’ve seen yet,” says James Kasting of Pennsylvania State University in University Park, who was not involved in the discovery.
The planet orbits a dim red dwarf star 20 light years from Earth called Gliese 581. Four planets were already known around the star, with two lying near the innerand outer edges of the habitable zone, where liquid water – and therefore potentially life – could exist on its surface.
One of those, which travels on a 13-day orbit, seems too hot for liquid water. The other, on a 67-day orbit, may be just warm enough for liquid water, but its status is not completely settled, says Kasting. Opinions “may continue to swing back and forth because it is hovering right near the outer edge”, he says.
The newly found “Goldilocks” planet, called Gliese 581 g, lies in between the hot and cold ones. “You’re smack dab in the middle of the habitable zone, so that’s perfect,” says Kasting, who has studied the two planets on the zone’s edges.
Steven Vogt of the University of California, Santa Cruz, and Paul Butler of the Carnegie Institution of Washington, DC, used the 10-metre Keck I telescope in Hawaii to measure the wobbles of the parent star in response to gravitational tugs from its planets. They combined their data with measurements published byMichel Mayor of Geneva Observatory and his colleagues using a 3.6-metre telescope at the European Southern Observatory in Chile.
The wobbles revealed two previously undiscovered planets around the star, for a total of six. One is about seven times the mass of Earth and in a 433-day orbit – much too far from its star to support liquid water.
The other, Gliese 581 g, lies in the habitable zone and has a 37-day orbit. Its mass is between 3.1 and 4.3 times that of Earth.
Its relatively low mass means it should be made mostly of rock, like Earth. Simulations show that planets that grow beyond about 10 Earth masses collect a lot of gas, becoming uninhabitable giants like Jupiter, with no solid or liquid surface to provide a toehold for life.
Some giant planets have previously been found in the habitable zones around their stars, but have generated less excitement because of their inhospitable structure.
Conditions on the planet would be very different from those on Earth. The host star is a low-mass red dwarf that is just 1 per cent as bright as the sun.
Because it puts out so little light and warmth, its habitable zone lies much closer in than does the sun’s. At such tight distances, planets in the zone experience strong gravitational tugs from the star that probably slow their rotation over time, until they become “locked” with one side always facing the star, just as the moon always keeps the same face pointed towards Earth.
That would mean perpetual daylight on one side of the planet and permanent shadow on the other. A first approximation suggests the temperature would be 71 °C on the day side and -34 °C on the night side, though winds could soften the differences by redistributing heat around the planet.
Travelling from one side of the planet to the other, there would be a range of intermediate temperatures, says Vogt. “The most comfortable place on this planet … is along what we call the terminator, the line between light and dark,” he says. “You basically see the star sitting on the horizon – you see an eternal sunrise or sunset.”
First of many
The discovery suggests habitable planets must be common, with 10 to 20 per cent of red dwarfs and sun-like stars boasting them, the team says. That’s because Gliese 581 is one of just nine stars out to its distance that have been searched with high enough precision to reveal a planet in the habitable zone.
“If you take the number of stars in our galaxy – a few hundred billion – and multiply them by 10 or 20 per cent, you end up with 20 or 40 billion potentially habitable planets out there,” says Vogt. “It’s a very large number.”
Although the new planet is in the habitable zone, we are unlikely to find out whether it is actually inhabited anytime soon. One way to find out would be to measure the planet’s light spectrum, which could reveal molecular oxygen or other possible signs of life in its atmosphere. But the overwhelming glare from its parent star makes it impossible to do that with current instruments.
However, Butler says this is probably just the first of many rocky planets likely to be found in the habitable zone in the coming years, and some of these are likely to be much more amenable to follow-up observations.