How To Find Our Planet’s Twin

What exactly constitutes an Earth doppelganger?

Planet twin

This artist’s impression shows the planet orbiting the Sun-like star HD 85512 in the southern constellation of Vela (The Sail). This planet is one of sixteen super-Earths discovered by the HARPS instrument on the 3.6-metre telescope at ESO’s La Silla Observatory. This planet is about 3.6 times as massive as the Earth and lies at the edge of the habitable zone around the star, where liquid water, and perhaps even life, could potentially exist.Courtesy ESO/M. Kornmesser/Nick Risinger

To date, most known extrasolar planets are gas giants like Jupiter or Neptune. Lacking even a solid surface, they are worlds very unlike our own planet. As the search for other Earths continue, a true Earth twin must satisfy five characteristics.

1) Earth size, with a rocky surface...

So far, astronomers have found only one planet with a measured size and mass similar to Earth’s: Kepler 78 b, announced last October. A recent study found that most planets smaller than one and a half times Earth’s diameter probably have rocky surfaces. Kepler has found more than 1,000 candidates, but they are not yet confirmed as Earth-like.

2) Near a sunlike star but not too close...

One in every five sunlike stars should have Earth-size planets in their habitable zones, according to a new analysis from the University of Cali-fornia, Berkeley and the University of Hawaii. But all Kepler’s rocky habitable-zone candidates circle dim red stars. Finding another Earth around another sun means watching the star for several years.

3) With liquid water...

Astronomers have found water in some exoplanet atmospheres, but identifying surface features is more difficult. The most promising technique is exocartography: Researchers map the planet’s surface by recording changes in color as oceans and continents spin past. Such data will require a five- to ten-meter space telescope.

4) Biosignatures...

Life on a planet will affect its chemistry. Detecting those signs will require taking spectra of the planet. Sara Seager, an astronomer at MIT, says Earth has so many biosignatures that astrobiologists don’t know which ones could prevail on a planet other than Earth. “We’re not going to find anything if we don’t keep an open mind,” she says.

5) ... And intelligent life

If aliens use infrared lasers to communicate across space—which isn't so crazy; Earthlings already do—the search for extraterrestrial intelligence (SETI) may make its first contact by intercepting those signals. This summer, scientists will begin a new SETI search using infrared detectors installed
at Lick Observatory in California.

Future Missions

Transiting Exoplanet Survey Satellite (TESS)
Launch: 2017
TESS will search for easy-to-see exoplanets around nearby stars across the whole sky. It's a shallower survey than Kepler's single-spot four-year stare, but it will cover much more ground. "It's going to pick all the low-hanging fruit," says Josh Winn, TESS's deputy science director.

James Webb Space Telescope
Launch: 2018
The Hubble heir apparent will be seven times as powerful and will study individual exoplanets with enough detail to learn about their atmospheres. It will also take pictures of some directly.

Starshades
Launch: Unscheduled
For truly detailed observations, astronomers will need to block the light of the star so they can see the planet more clearly. Multiple teams are developing a space-based starshade that would align itself to an orbiting telescope precisely enough to hide a star and reveal the planet.

This article originally appeared in the February 2014 issue of Popular Science.