If finding Earth-size planets orbiting other stars at distances hospitable for life is the holy grail for planet hunters, teams of astronomers are uncovering a lot of potential chalices.
One team, associated with NASA's Kepler mission, announced Monday that they have identified 1,781 planet "candidates" as they peer at a patch of sky covering more than 156,000 stars, in the constellation Cygnus.
Of those 121 appear to be orbiting in the habitable zones of their host stars – a region of space around the star where a planet's temperature would allow liquid water to pool on the surface. Some of these habitable-zone objects appear to be so-called super Earths, which tip the scales at between roughly twice Earth's mass to 10 times the mass of the Earth.
The Kepler team calls the objects candidates because they still require confirmation from ground-based astronomers using other techniques for detecting them. The team announced the latest update to its planetary census at a conference in Moran, Wyo., near Grand Teton National Park.
Kepler, a spacecraft trailing behind Earth in the planet's orbit around the sun, stares at its target stars simultaneously. It uses the so-called transit method to find planets, looking for the telltale dimming as a planet passes in front of its host star. To confirm that the observed object is a planet, ground-based astronomers look at the impact of the planet's motion on the star's spectrum. Of the 1,781 candidates Kepler has detected, 27 have been confirmed as planets so far.
Although the detection method Kepler uses allows scientist to make some broad inferences about a planet's size, temperature, or whether the planet is rocky or a ball of gas, the planets are too far away to make direct measurements of any atmosphere they might have.
The stars Kepler looks at are roughly 600 to 3,000 light-years away.
That is why other teams are looking much closer to home.
A group using a planet-hunting spectrometer bolted to the back of a telescope at the European Southern Observatory's La Silla Observatory in Chile reported at the same conference finding a super Earth orbiting just inside the inner edge of the habitable zone of a star labeled HD 85512. It is in the constellation Vela some 6.2 light-years from the sun.
The planet, with 3.6 times Earth's mass, orbits the star once every 58 days. By comparison, Mercury orbits the sun once every 88 days.
The team detected the planet with a technique that uses the wobble in a star's spectrum to track the planet's gravitational tug on its sun. The discovery was part of a project to see if upgrades to the spectrometer they are using, dubbed HARP, was up to the task of detecting planets with masses comparable to Earth's in the habitable zones of near-by stars.
The planet has the lowest mass of any yet detected in a star's habitable zone, according to Lisa Kaltenegger, a planetary scientist who divides her time between the Max Planck Institute for Astronomy in Heidelberg, Germany, and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
The detection was one of 50 new confirmed planets – including 16 super Earths – HARP has added to the roster of what is now 645 planets astronomers around the world have discovered.
While the number of planets approaching Earth's mass and within a star's habitable zone is increasing, distance alone does not make for a comfy planet, researchers caution.
In August, Dr. Kaltenegger and Dimitar Sasselov, with the Harvard-Smithsonian Center for Astrophysics, published basic calculations that could be applied to the Kepler data to give a first-cut estimate of whether a planet candidate in a habitable zone could indeed be habitable.
The breadth of a star's habitable zone depends as much on the planet itself as it does on the star, the scientists point out.
Among the key factors they cite: the intensity, mix of wavelengths, and steadiness with which the star's light hits the planet; the amount of starlight the planet reflects; the concentration of heat-trapping greenhouse gases; and the range of geophysical processes, from volcanism to wind patterns, that move energy around.
Since most of those characteristics are not observable, researchers by necessity have to make some assumptions about the traits, especially those the planets display.
When the two applied their model to Kepler data, they found fewer desirable neighborhoods than initially estimated.
In February, the Kepler team announced that the latest update to its planetary census revealed 54 planet candidates falling within their stars' habitable zones.
But based on the traits Kaltenegger and Dr. Sasselov identified, the duo estimated that many of the 54 planets actually fall outside of the habitable zones. Of the six likely rocky planets in that sample, four fell within the zone, with two others on the inside fringe. Think Venus.
Close, but no microbe.