With today's technology, analysis on an exoplanet's atmosphere is only possible if the world passes in front of its parent star. NASA, ESA, and G. Bacon (STScI)
— Discovery of an Earth-like planet in a habitable zone was found after looking at just nine nearby stars.
— Such a small sampling implies that worlds like Earth are extremely common.
— Techniques are under development to look for the chemical fingerprints of life in the atmospheres of such planets.
A new chapter in the search for life beyond Earth opened this week with the discovery of the first rocky world well positioned around its parent star to hold pools of water, an environment believed to be necessary for life.
Due to an unfortunate viewing angle, scientists can't determine if this particular planet, known as Gliese 581 g, is indeed endowed, but they don't expect to have to wait too long to find similar worlds that can be probed.
The key is finding planets that pass in front of their parent stars, relative to Earth's line of sight. By tracking the minute changes in the light as the planet circles in front of, and then behind, its star, scientists can ferret out which tiny bit of radiation is coming from the planet amidst the huge amount of light streaming from the star.
"When (the planet) goes behind the star, the planet light is not involved, and when it comes back it's there, so you can separate it out," Sara Seager, a planetary scientist at the Massachusetts Institute of Technology, told Discovery News.
If life exists, scientists expect the planet's rays to be embedded with tell-tale chemical fingerprints of atmospheric molecules, such as oxygen and methane. Decoding the chemistry is challenging, since many factors, such as temperature and molecular interactions, have an impact. But even more daunting is figuring out what chemistry stems from the metabolisms of life and what is tied to geologic activities, such as volcanic eruptions.
"The difference between seeing things that are due to life and knowing that what you're seeing must be due to life sounds technical, but it's a humongous challenge for science," Eric Ford, an associate professor of astronomy at the University of Florida, told Discovery News.
The starting point is finding atmospheric molecules that have no business being there if they weren't being replenished by something on the planet's surface. Oxygen, for example, breaks down in ultraviolet light, so if it's found in abundance in a planet's atmosphere, that's a huge flag, since on Earth, oxygen comes from plants and photosynthetic bacteria.
"You look for signs where the chemistry has been perturbed from equilibrium conditions," said Mark Swain, an exoplanet research scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
The science of exoplanet atmospheres is in its infancy. Swain says just four extrasolar planets' atmospheres have been studied in detail with spectroscopy, a chemical analysis technique. Another 32 or so have been observed.
"Part of what took so long is just figuring out how to do it," Swain told Discovery News.
None of those planets are like Gliese 581 g, a rocky world about three times the mass of Earth orbiting — along with five sibling planets — a red dwarf star 20 light-years from Earth.
Assuming scientists didn't hit the cosmic jackpot, the discovery of Gliese 581 g — from a pool of just nine nearby stars — implies that Earth-like worlds in life-friendly zones are extremely common.
"This is right next door to us," said astronomer Steven Vogt, with the University of California Santa Cruz, who announced the planet's discovery Wednesday, along with collaborator Paul Butler, with the Carnegie Institution in Washington, D.C.
"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," Vogt said. "There could be tens of billions of these systems in our galaxy."
Scientists caution that just because Gliese 581 g is the right temperature to hold liquid water doesn't mean that it has any.
Venus, for example, lost its water, and Mars' water — what's left of it — is locked up in its soil.
"It's a little bit of an assumption to say that this planet, just because it's in the right place, has water," Swain said.
Vogt acknowledges its speculative to say Gliese 581 g has water, but he adds that water is found just about every place you look — on the moon, Mars, beneath the frozen crust of Jupiter's moon Europa, blasting out of volcanoes on Saturn's moon Enceladus, and free-flying in space.
"There's enough water produced in the Orion Nebula every 24 seconds to fill the Earth's oceans," Vogt said. "It's pretty hard to imagine that water wouldn't be on Gliese 581 g."
On Earth, any place that has water has life, Vogt noted.
"It's pretty hard to stop life once you give it the right conditions," he said.