In search for life, more planet 'candidates' are found. Are any just right?

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.

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