First Exoplanet With CO2 Heats Up Hunt for Other Earths

Anne Minard

The recent discovery of carbon dioxide in the atmosphere of a Jupiter-like planet 63 light-years away has some researchers excited that we may soon find habitable exoplanets—worlds circling other stars.

According to lead author Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, California, carbon dioxide is a biomarker, a molecule associated with life as we know it.
This first discovery of the molecule on a far-flung planet, he said, is a step toward eventually finding biomarkers on smaller, more Earthlike worlds.

Last March, Swain and colleagues had announced the first detection of another biologically important chemical, methane, on the same exoplanet, as well as confirmation of water vapor.

"Methane is a potential marker [of life], as is water, as is carbon dioxide," Swain said. "So, three of the biggies we've already detected."

But not all planetary scientists are embracing the idea that finding chemicals such as carbon dioxide and methane on a distant planet is enough to say it is habitable.

On Earth, for example, carbon dioxide traps a certain amount of sunlight, keeping temperatures warm enough to support life. But on Venus, too much carbon dioxide creates a killing heat—average temperatures there reach 864 degrees Fahrenheit (462 degrees Celsius).

"To me, a CO2-dominated atmosphere says the planet would certainly be more like Venus than Earth," said Ellen Stofan, a planetary geologist for Proxemy Research in Maryland and a member of NASA's Cassini science team.

"To say it precludes life or guarantees life are both going out on a limb—we just don't know. But it is intriguing!"

Free Oxygen

Swain and colleagues made the CO2 discovery while studying radiation from an exoplanet dubbed HD 189733b.

The planet periodically transits—or passes in front of—its host star, giving observers a measurement of the star and planet's combined light. By subtracting the light from the star alone from the combined measurement, astronomers can "see" the planet's light.

Reading signatures from this light then told astronomers the chemical composition of the planet's atmosphere.
Swain acknowledges that HD 189733b is too big, gaseous, and hot to host life as we know it.

But the fact that we are now able to find biomarkers such as carbon dioxide on worlds so far away means that we'll know what to look for when reading light signatures from smaller, rocky planets like Earth.

So far, the most elusive biomarker on any exoplanet has been oxygen.

The only thing that drives the presence of free oxygen molecules on Earth is plants, points out Geoff Marcy, an astronomer at the University of California, Berkeley, who was not involved with the carbon dioxide study.

Without plants to generate O2, all the oxygen in the atmosphere would eventually react with other elements and be trapped.

"If you could detect free oxygen [on an exoplanet], then you'd have something. That would be remarkable," Marcy said.

Planet-Hunter's Toolkit

Astronomers searching for habitable exoplanets must first face the daunting challenge of finding distant Earthlike worlds.

Astronomers think at least half of the sunlike stars in our galaxy could host habitable terrestrial worlds.

But most of the more than 330 exoplanets found since the first discovery in 1995 have been large gaseous bodies that resemble Jupiter. Plenty have also been detected that are closer to the size of Saturn or Neptune, down to 17 times the mass of Earth.

Planets down to about five times the mass of Earth have been much rarer discoveries, and Berkeley's Marcy believes rocky planets—the holy grail, he calls them—are likely to be no more than three times the mass of Earth.

For now planetary detection methods are best suited to spotting larger bodies.

So far 270 of the known exoplanets have been detected using the so-called Doppler wobble, a shift in light from the parent star caused by a massive planet's gravitational tug.

About 50 or 60 exoplanets have been found because they transit their parent stars, and about 5 worlds have been spotted using a bizarre physical phenomenon predicted by Einstein's theory of relativity: gravitational lensing.

This is when light from other stars is bent by the gravitational tug of an object in the foreground that is orbiting a star too faint for us to see from Earth.

Marcy has high hopes that NASA's upcoming Kepler mission will bring the quest for rocky exoplanets closer to its goal.

"It's the hottest thing going, there's nothing even close to it," said Marcy, who is part of the Kepler science working group.

Set to launch on March 5, the space telescope is designed to find other Earths by staring at the constellation Cygnus and detecting stars that dim due to transiting planets.

Cygnus was chosen because it contains a large number of stars and won't be obscured by the sun at any time of the year.

The probe's photometers are sensitive enough to catch minute changes in light that result during a transit by a small planet, and its "eye" can see a hundred thousand stars at once.

"If Kepler finds that most stars have a terrestrial planet in the habitable zone, then there must be billions of such planets in our galaxy and life could be ubiquitous," said William Borucki, Kepler mission lead scientist at NASA.

"On the other hand, if Kepler finds no terrestrial planets or extremely few, then life of any type must be very rare. We could be the only sentient life."

Planet Trek

Even if Kepler proves there are scores of habitable worlds within Cygnus, the constellation contains stars that are a hundred to more than a thousand light-years away.

That means there won't be much to do but point radio telescopes toward them and scan for spillover from alien communication technologies.

(Related: "Are Neighborhood Aliens Listening to Earth Radio?" [September 7, 2006].)

That's because travel to any of its planets using current propulsion methods wouldn't be possible, at least not in a single generation, Marcy noted.

The idea requires acceptance of "having your great grandchildren be the ones who arrive at Alpha Centauri, not you."

Meanwhile, Notre Dame's Bennett points out that heavy elements—the main ingredients for planetary systems—are most common near the center of the galaxy, which is billions of years older than our solar system.

This ups the chances that any alien civilizations are much more advanced technologically than humans.

"If they're a few billion years ahead of us," Bennett said, "it's more of a question of them getting here than us getting there."