February 14, 2008

Rocky Planets May Orbit Most Stars

Astronomers have discovered that rocky, terrestrial planets might orbit many, if not most, of the nearby sun-like stars in the disk of our galaxy. These new results suggest that worlds with potential for life are more common that we thought.
This release is being issued jointly with the University of Arizona (UA).
UA astronomer Michael Meyer (currently on sabbatical at the Harvard-Smithsonian Center for Astrophysics) used NASA's Spitzer Space Telescope to determine whether planetary systems like ours are common or rare in the Milky Way galaxy.
Meyer and his colleagues found that at least one-fifth (20 percent), and possibly as many as three-fifths (60 percent), of stars similar to the sun are candidates for forming rocky planets.
Meyer discussed the findings at a news briefing held in Boston yesterday (Feb.17) in conjunction with the annual meeting of the American Association for the Advancement of Science. The results were published in the Feb. 1, 2008, issue of Astrophysical Journal Letters.
The astronomers surveyed six groups of stars with masses comparable to our sun. The stars were grouped by age, ranging from three to 10 million years, 10 to 30 million years, 30 to 100 million years, 100 to 300 million years, 300 million to one billion years, and one to three billion years old.
"We wanted to study the evolution of the gas and dust around stars similar to the sun and compare the results with what we think the solar system looked like at earlier stages during its evolution," Meyer said. The sun is about 4.6 billion years old.
The Spitzer telescope can detect cosmic dust at a range of temperatures. The astronomers looked for hot dust (at temperatures up to 2,000 degrees Fahrenheit), warm dust (between -280 and 80 degrees F), and cold dust (from -380 to -280 degrees F).
Because dust closer to the star is hotter than dust farther from the star, the warm dust likely traces material orbiting the star at distances comparable to distances between Earth and Jupiter around our star, the sun.
"We found that about 10 to 20 percent of the stars in each of the four youngest age groups show emission due to warm dust," Meyer said. "But we don't often see warm dust around stars older than 300 million years. The frequency just drops off.
"That's comparable to the time scales thought to span the formation and dynamical evolution of our own solar system," he added. "Theoretical models and meteoritic data suggest that Earth formed over 10 to 50 million years from collisions between smaller bodies."
Scott Kenyon, of the Smithsonian Astrophysical Observatory, and Ben Bromley, of the University of Utah, have developed planet formation models that complement Meyer's work. Their models predict warm dust would be detected during planet formation, as small rocky bodies collide and merge, creating larger rocky bodies and eventually assembling the asteroids, moons and planets.
Kenyon said, "Our work suggests that the warm dust Meyer and colleagues detect is a natural outcome of rocky planet formation. We predict a higher frequency of dust emission for the younger stars, just as Spitzer observes."
The numbers on how many stars form planets are ambiguous because there's more than one way to interpret the Spitzer data, Meyer said.
The warm-dust emission that Spitzer observed around 20 percent of the youngest cohort of stars could persist as the stars age. That is, the warm dust generated by collisions around three to 10-million year old stars could carry over and show up as warm dust emission seen around stars in the 10 to 30-million year old range and so on.
Interpreting the data this way, at least one out of five sun-like stars is potentially planet-forming, Meyer said.
There is another way to interpret the data.
"An optimistic scenario would suggest that the biggest, most massive disks would undergo the runaway collision process first and assemble their planets quickly. That's what we could be seeing in the youngest stars. Their disks live hard and die young, shining brightly early on, then fading," Meyer said.
"However, smaller, less massive disks will light up later. Planet formation in this case is delayed because there are fewer particles to collide with each other," he said.
If this is the correct interpretation - that the most massive disks form their planets first and the wimpiest disks take 10 to 100 times longer - then as many as 60 percent of the surveyed stars have formed, or may be forming, planets.
"The correct answer probably lies somewhere between the pessimistic case of less than 20 percent and optimistic case of more than 60 percent," Meyer said.
The next critical test of the assertion that terrestrial planets like Earth could be common around stars like the sun will come next year with the launch of NASA's Kepler Mission. Kepler will detect the tiny dips in the amount of light seen as planets pass in front of their stars.
The Giant Magellan Telescope, one in a new generation of extremely large ground-based telescopes, could also play a role in determining how many neighboring sun-like stars are forming planets, Meyer said.
The Giant Magellan Telescope, known as the GMT, is slated for completion in 2016 at a site in northern Chile. It will use seven 8.4-meter primary mirrors arranged as a hexagon on a single mount. The telescope will have the resolution, or light-resolving power, equivalent to a telescope with a single 24.5-meter, or 80-foot diameter, primary mirror.
UA astronomer Phil Hinz, who has been pioneering adaptive optics systems to improve seeing with the new giant ground-based telescopes, said, "Imaging and spectroscopically characterizing an Earth-like planet is an incredibly difficult task. But if Earth-like planets really are this common, we might just be able to take the first picture of another Earth with the GMT. It's an exciting prospect!"
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.