Two decades ago, astronomers found the first planet orbiting a star like our sun, a giant world that lies about 50 light-years away. That discovery opened a floodgate, and today scientists have pinpointed more than 1800 extrasolar planets—or exoplanets—in a variety of shapes, sizes and compositions. The big questions now are how common planets like ours might be in our galaxy, and how many worlds like Earth have the right stuff to host life.
Figuring out the best places to look often involves searching for exoplanets that appear to have the same rocky composition as Earth and that orbit just far enough away from their stars to have liquid water on their surfaces. Now two teams at the Harvard-Smithsonian Center for Astrophysics (CfA) have refined the hunt by calculating the sizes and masses of worlds most likely to be friendly to life.
Bolstering that theoretical work, astronomers also announced this week that 12 worlds discovered by NASA's Kepler space telescope are most likely small planets akin to Earth that orbit in the habitable zone, the region around a star that's just right for water, and perhaps life. The work doubles the number of known Earth-sized worlds that may be habitable.
"We don't have an exact Earth twin yet, but we are starting to fill in the population of planets that are in the neighborhood," says Kepler scientist Douglas Caldwell at the SETI Institute. Caldwell presented the team's analysis of the 12 planets today at the 225th meeting of the American Astronomical Society in Seattle.
Technically, only 11 of the dozen worlds have been verified to the same level of confidence as other confirmed Kepler planets. The 12th world is still considered a candidate planet, although Caldwell says the team's confidence that it is real is still very high. Of the 11 confirmed planets, the two smallest are especially interesting in the hunt for Earth's twin. Kepler-438b is a mere 1.12 times the size of Earth, while Kepler-442b is 1.33 times Earth's size.
That means both planets are highly likely to be rocky worlds with the same chemical composition as Earth, according to a separate study led by the CfA's Courtney Dressing. In our solar system, planets are either small and rocky, like Earth and Mars, or big and gassy, like Jupiter and Neptune. Much to astronomers' surprise, plenty of the exoplanets found so far are strange objects that fall between the sizes of Earth and Neptune. We can't yet see these exoplanets directly—astronomers found them by looking for the slight dip in starlight as the orbiting planets pass in front of, or transit, their host stars. Without a more detailed view, scientists have not been sure which "super Earths" are truly rocky and which are more like icy mini-Neptunes.
"Part of the problem with figuring out whether super-Earths are rocky or gaseous is that we don’t have any examples in own solar system," says Caldwell. To tease out an answer, astronomers must get both the size and the mass of a planet and calculate its density, which gives an idea of its composition. So Dressing and her team turned to a telescope in the Canary Islands that's fitted with an instrument to measure planet masses with high accuracy. They used it to study several known planets and chart the relationship between size and mass.
They found that smaller worlds less than 1.6 times Earth's size tend to have the right masses for the planet to be rocky. What's more, most of the small exoplanets they examined had the right density to have about the same ingredients as Earth: a mix of silicon, iron, oxygen, magnesium with trace amounts of other metals. Larger worlds are usually much less dense, meaning they must be mainly hydrogen or other gasses.
"Our solar system is not as unique as we might have thought," Dressing says in a statement. "It looks like rocky exoplanets use the same basic ingredients."
Another main factor is that life as we know it seems to depend on large bodies of liquid water. On Earth, water covers roughly 70 percent of the planet's surface. But recent work shows that it also moves through the interior of the planet, as it is driven underground by plate tectonics, trapped in minerals and spewed back out by volcanoes. Researchers now think this watery recycling process is crucial to keeping oceans stable on planetary surfaces.
Laura Schaefer and Dimitar Sasselov at the CfA used computer models to see how well exoplanets up to 1.5 times the size of Earth can establish and maintain plate tectonics. They varied the masses of their possible worlds, going up to five times as massive as Earth. The results suggest that super-Earths between two and four times Earth's mass would be best at building stable oceans. The seas on these worlds would last for at least 10 billion years, they say.
But their model also showed that more massive worlds have thicker crusts, which delays the start of volcanic activity, and thus ocean formation, at the surface. They calculate that it takes about a billion years after such a massive planet forms for an ocean to develop. Assuming evolution follows a similar rate as it has on Earth, our best bet for finding a planet with life may be a super-Earth that's at least 5.5 billion years old, the team says.
The new Earth-sized planets found by Kepler orbit stars that are relatively young. And for now scientists are not sure about the masses of these planets. But simply having more planets in the catalog that are at least cousins to Earth helps astronomers answer the question of how common potentially habitable worlds are in our galaxy.
"Everyone wants to be the first to find and announce Earth's twin, but scientifically that won't be the most important thing that comes out of Kepler," says Caldwell. "With super-Earths, we're seeing a class of planet that we didn’t know about before, so seeing how they formed and how different they are from Earth can improve our models of how all planets form. How did water get on Earth, and when in the formation process did it get here? By finding similar planets in systems at other ages, we're hoping to get a better idea on that question."
Editor's Note: This story has been updated to correct the date for the first exoplanet found around a sunlike star; that discovery was made two decades ago, in 1995.