'Seven Sisters' star may be giving birth to planets

Rocky planets the size of Earth or Mars may be forming around a sun-like star in the Pleiades star cluster, astronomers said this week.

Writing in Wednesday's Astrophysical Journal, U.S. astronomers said an unusually large number of dust particles surrounding the star HD 23514 suggest the aftermath of a collision of early planetoids.

Stars spew forth large amounts of material during the first 10 million years of their existence, said co-author Inseok Song, a staff scientist at NASA's Spitzer Science Center at the California Institute of Technology.

But at the age of the star observed between 100 million and 400 million years the dust should have either clumped together to form planets and comets or been sucked back into the star, Song said.

"Unusually massive amounts of dust, as seen at the Pleiades and Aries stars, cannot be primordial but rather must be the second-generation debris generated by collisions of large objects," Song said in a statement.

The planets in our solar system are thought to have formed in a similar manner, as larger and larger masses collided with each other. Our sun, for example,is estimated to beabout 4.6 billion years old.

The findings come from observations made using two telescopes: the Spitzer Space Telescope, which provided infrared images, and the Hawaii-based Gemini North telescope, which measured the heat radiation coming from the dust.

UCLA researcher Joseph Rhee, the lead author of the study, said the results "may well be the first observational evidence that terrestrial planets like those in our solar system are quite common."

Visible from the night sky, the Pleiades, or the Seven Sisters, take their name from the seven daughters of Atlas and Pleione of Greek mythology. But the star cluster, which lies about 400 light years away, actually contains some 1,400 stars.

Astronomers have so far discovered more than 250 extrasolar planets, or "exoplanets."

The bulk of these have been gas-giant planets similar in size to Jupiter, which are easier to spot because their large mass has a greater gravitational influence on the stars they orbit, thus allowing astronomers to infer their existence even when they cannot directly "see" them.

Earth-size planets have proved more difficult to find, though earlier this year scientists made headlines when they found evidence of an already-formed Earth-like planet around the star Gliese 581, a star 20.5 light years away, or 1.93 hundred million million kilometres away.