Planet cousins

A new-found planet displays attributes similar to the ones found on our Earth. So where does this leave all the other numerous claims of finding earth-like planets, asks Dan Vergano

Red sunshine, seas and maybe aliens? Scientists analysing data from NASA’s Kepler Space Telescope report the closest thing yet to another Earth, a world in a habitable orbit around a red dwarf star some 493 light-years away.

Launched in 2009 with the goal of finding another Earth, the $600-million Kepler spacecraft has discovered more than 960 planets orbiting nearby stars. Half a dozen of those seem to be rocky, like Earth, and have orbits in the habitable zone around their star, but the newly discovered world, named Kepler-186f, is the closest in size to Earth. “This is a first, validated Earth-size planet in the habitable zone of another star,” says study lead author Elisa Quintana of the SETI Institute and NASA’s Ames Research Center in Mountain View, California.

One of five planets orbiting a red dwarf star (called Kepler 186), Kepler-186f is 1.1 times wider than Earth. That means it’s almost certainly a rocky planet too. The researchers estimate its mass is 1.5 times that of Earth’s. The new planet’s orbit, meanwhile, places it at the “Goldilocks” distance from its star – not too hot or too cold for liquid water to exist on its surface. The origin of life on Earth required liquid water, notes study co-author Stephen Kane of San Francisco State University.

“This is an historic discovery, the first Earth-size planet found in the habitable zone around its star,” says pioneering planet hunter Geoff Marcy of the University of California, Berkeley, who was not on the discovery team. “This is the best case for a habitable planet yet found.”

The planet’s red dwarf star is only about half as big as the sun, making it cooler and dimmer. But Kepler-186f is on a tighter orbit than Earth is, taking only 130 days to circle its star. Though it receives less warmth from its sun than Earth does from its own, the discovery team says, it would still be warm enough to prevent seas from freezing, provided it has an atmosphere that provides a substantial greenhouse effect. “This planet basks in an orange-red glow from that star, much as we enjoy at sunset,” Marcy says, by email. “The temperature on the planet is likely cool, similar to dawn or dusk on a spring day.”

Crowded claims

“Sounds like a great planet to visit, if we could figure out how to travel there,” says MIT astronomer Sarah Seager, by email. But amid the excitement, she and planetary scientist Alan Boss, author of “The Crowded Universe: The Search for Living Planets,” caution that other discoveries have led to similar claims in recent years.

Since 1995, astronomers have detected nearly 1,700 worlds orbiting nearby stars, using a variety of detection methods. About a half dozen claims of bigger Earth-size planets orbiting in habitable zones around red dwarfs have been made in recent years, Boss says.

The next closest thing to Kepler-186f has a width 1.4 times that of Earth, Quintana says. According to Seager, a planet whose diameter is less than 1.75 Earths is likely to be rocky. The Kepler report looks particularly reliable because of the spacecraft’s track record. It detects planets that dim the light from their stars as they pass in front of them. Such transits, Quintana says, are observable only in the roughly one percent of planetary systems whose orbits can be seen edge on from Earth.

When transits occur regularly, their frequency allows scientists to calculate the distance at which a planet is orbiting a star. The amount of starlight dimming – typically on the order of 0.1 percent is a measure of the planet’s size.

Such searches are most sensitive to closer-in stars, because fewer days of observations are required to see repeated transits. That explains why the newly discovered planet’s four closer-in siblings had been spotted earlier by the space telescope. “They relied on only two years of data,” Quintana says. With so many planets in the system, it’s likely to be stable over billions of years.

Tickets on hold

Whether a life-friendly atmosphere exists on Kepler-186f depends on a bevy of factors besides having the right orbit. “We see planets in our own solar system, Venus but also Mars that are Earth-like but where things didn’t work out,” Kane says.

On Venus, a runaway greenhouse climate has cooked the surface to temperatures that would melt lead. On Mars, the lack of a strong magnetic field has allowed the solar wind to strip away much of the planet’s atmosphere. A magnetic field would be particularly important for a planet orbiting a red dwarf, because such stars tend to release strong flares that would sterilise the planet. “Just because a planet is in the habitable zone doesn’t mean it is habitable,” Quintana says. “This is sort of a first step.”

However, Kane argues that the greater mass of Kepler-186f makes it more likely than Mars to have an interior heated by radioactivity and stirred by the motion of fluids. Such motions are required to power a dynamo that generates a protective magnetic field as well as volcanoes, whose eruptions would help replenish a life-friendly atmosphere. The planet’s mass would also give it enough gravity to hold on to that atmosphere.

“The other big question is whether it has water, delivered by comets or some other means,” Kane says. “Any place with liquid water is a natural place to look for life.” Unfortunately, Kepler-186f is likely too dim and far away to be seen directly with any telescope now in operation, or even with NASA’s James Webb Space Telescope, scheduled for launch in 2018.

“In reality, we cannot know if the planet is actually habitable. We need to get a sense of the atmosphere and its greenhouse effect,” Seager says. “Not possible for this particular planet, as it is too distant from Earth for follow-up observations.”

Kepler’s chase

The latest Kepler discovery came from a trove of star observations that the spacecraft made before a reaction wheel in its steering system failed last year, hobbling the mission.
A reduced “K2” mission was announced in March. Hiding amid the existing Kepler observations, Kane says, are more unconfirmed “candidate” planets orbiting stars as big as the sun, at distances similar to Earth’s 93-million-mile (150-million-kilometer) distance from the sun. “There are still a lot more Kepler ‘habitable zone’ worlds out there to find,” Kane says. “Almost certainly this is not the last one.