EDITORIAL: In the Habitable Zone Published: September 30, 2010

A planet orbiting a star causes a slight disturbance in the star’s rotation, the effect of the gravitational tug between the star and the planet. Astronomers have been studying the wobbling of stars for a couple of decades, in hopes of finding an exoplanet→ — a planet beyond our solar system — that might offer the possibility of sustaining human life. Now, after 11 years of searching with specialized instruments in Chile and Hawaii, a team of American astronomers has announced in the Astrophysical Journal that it has found the first likely candidate.

The planet is called Gliese 581g after its sun, Gliese 581, a red dwarf vastly dimmer than our sun and about 20 light-years from Earth. Potentially habitable does not mean Earthlike. It means that Gliese 581g is the right distance from its sun to be in the habitable zone, able to sustain liquid water and with enough gravity to retain an atmosphere. Gliese 581g orbits its sun in a bit more than 36 days and is almost certainly tidally locked, meaning the same side of the planet always faces the sun. That probably means wide extremes in temperature and a permanent twilight zone between night and day where the climates are more moderate.

What makes this discovery so important is that it happened so early in the search for exoplanets and after examining only a tiny sample of small candidate stars as close to Earth as Gliese 581. In the paper reporting their discovery, the astronomers → discuss the probable implications with carefully calibrated language that still doesn’t hide their excitement. “If the local stellar neighborhood,” they write, “is a representative sample of the galaxy as a whole, our Milky Way could be teeming with potentially habitable planets.” We are intrigued, too.

A version of this editorial appeared in print on October 1, 2010, on page A24 of the New York edition.

http://www.nytimes.com/2010/10/01/opinion/01fri4.html?scp=1&sq=in%20habitable%20zone&st=cse