A clone of Earth, or even a rough approximation, has proved a difficult thing for scientists to find. The catalogue of planets orbiting other stars grew to more than 400 entries in October, but the goal that drives much of the research into extrasolar planets, or exoplanets, is the discovery of a habitable world, and that goal remains unmet. Some rough approximations, however, are starting to come into view.

Based on humankind's admittedly limited experience, habitability seems to mean a small world—a terrestrial planet rather than a gas giant like Jupiter or Saturn—orbiting its star at a comfortable "Goldilocks" distance that allows water to persist in liquid form. It is unfortunate, then, that some of the easiest planets to detect are the so-called hot Jupiters: massive bodies hugging tight to their host stars and therefore subject to extremely high (and probably life-negating) temperatures.

But a trio of papers published or announced this week shows just how far exoplanet searches have come and how such projects are progressing toward finding an Earth-like planet, possibly even a nearby one. In the December 17 issue of Nature, David Charbonneau of the Harvard–Smithsonian Center for Astrophysics and his colleagues present evidence for a nearby so-called super-Earth—a planet bigger than our own but smaller than Uranus and Neptune, the next largest planets by mass and diameter, respectively, in the solar system. (Scientific American is part of Nature Publishing Group.) And on Monday a pair of studies set to be published in the Astrophysical Journal were unveiled, each claiming the discovery of a different super-Earth orbiting nearby sunlike stars.

Earth analogues, and to a lesser extent super-Earths, have been so elusive in part because the two primary modes of exoplanet detection favor larger, hotter planets. Transit searches, which track the periodic dimming of a star as a planet passes in front of it, more easily detect large planets in close orbits that blot out more light when they eclipse their stars. And radial velocity searches, which look for Doppler shifts in a star's light as it wobbles under the influence of an orbiting companion, are more attuned to massive planets that induce greater gravitational wobbles in their host stars.

The planet found by Charbonneau and his colleagues, known as GJ 1214 b, has an estimated mass of 6.55 Earths and a diameter 2.68 times that of Earth. Every 38 hours GJ 1214 b completes an orbit around GJ 1214, a dim, lightweight red star known as an M dwarf that lies just 42 light-years from the sun.

GJ 1214 b was discovered by the MEarth Project, a campaign using eight amateur-size telescopes to monitor nearby M dwarfs for periodic dimming caused by transiting planets. The amount of observed dimming, in conjunction with the known size of the star, allowed Charbonneau's group to estimate GJ 1214 b's diameter. The MEarth team also used radial velocity data from a larger telescope to detect the planet's gravitational pull on its host star, which yielded a mass estimate. After COROT 7 b, discovered earlier this year by the space-based European COROT telescope, GJ 1214 b is the smallest planet whose diameter and mass are known.

M dwarfs are particularly appealing targets because of their cool temperature. Even a planet like GJ 1214 b, which follows an orbit much tighter than any found in the solar system, might be nearly cold enough for water to condense into liquid form, provided it had an atmosphere similar to Earth's. The newfound planet is "a little bit too hot to have liquid water, but it didn't miss it by much," Charbonneau said in a telecast news conference Monday. "This is probably not what you would call a habitable world," he cautioned, pointing out that such a discovery is likely just a few years away. NASA's Kepler mission, and to a lesser extent COROT, is well-primed to find Earth analogues, provided they are relatively common, as astronomers suspect.

The hot little planet may have liquid water anyway, Charbonneau said: its density is consistent with a water-heavy makeup, plus a crushing atmosphere whose pressure would allow water to remain liquid at high temperatures. But exoplanet researcher Sara Seager, a Massachusetts Institute of Technology astrophysicist who did not participate in the new study, notes that "we don't really know" what GJ 1214 b is made of; a number of ingredient distributions would account for the planet's observed density. "There's no unique combination" that yields a given density, she says. The relative closeness of GJ 1214 b to the solar system should allow more detailed study by large telescopes already in operation. "What we hope to do in the very near future is learn about the atmosphere directly," Charbonneau said.

Rounding out the super-Earth triad
In one of the Astrophysical Journal papers released this week, astronomer Steven Vogt of the University of California, Santa Cruz, and his colleagues found an even less massive super-Earth around 61 Virginis, a star just 28 light-years away that is approximately sunlike in mass, diameter and luminosity. An estimated 5.1 Earth masses, 61 Vir b appears to be accompanied by at least two larger planetary siblings in the 61 Virginis system.

The third super-Earth unveiled this week comes from a subset of the group that uncovered the planets around 61 Virginis. The smaller team makes the case for at least one, and possibly three, planets orbiting the sunlike star HD 1461, some 76 light-years distant. The more solid planetary detection is the super-Earth HD 1461 b, with an estimated mass 7.4 times that of Earth.

Both 61 Vir b and HD 1461 b were detected by radial velocity, or stellar wobble, measurements alone, meaning that their diameters remain unknown. Follow-up observations may be able to spot planetary transits, but only if the planets' orbits carry them past their respective stars along our line of sight. In any case, given that 61 Vir b and HD 1461 b orbit sun analogues at roughly one sixth the distance that Mercury circles the sun, both would be scalding bodies inhospitable to life as we know it.

Although habitable worlds remain elusive, astronomers are steadily passing the milestones along the way. "Until now, the planets we've been able to study in detail are the hot Jupiters," Seager says, citing the new announcements as a sign of things to come. "The new year coming up is kind of like a new year for us in exoplanets, because we're shifting from the old stuff—hot Jupiters—to the new stuff, which is super-Earths."