18 February 2008
Plenty Of Earth-Like Planets Out There, Say Astronomers
by Kate Melville
Terrestrial planets might form around many, if not most, of the nearby sun-like stars in the disk of our galaxy, believe University of Arizona astronomers. Revealing the findings to attendees at the annual meeting of the American Association for the Advancement of Science, team leader Michael Meyer said that at least 20 percent, and possibly as many as 60 percent, of stars similar to the sun are candidates for forming rocky planets.
The astronomers surveyed six groups of stars with masses comparable to our sun using the Spitzer Space Telescope. 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 Spitzer telescope detects dust at a range of infrared wavelengths. The hottest dust, at temperatures more than 2,000 degrees Fahrenheit, is detected at the shortest wavelengths, between 3.6 microns and 8 microns. Cool dust, about minus 380 degrees Fahrenheit, is detected at the longest wavelengths, between 70 microns and 160 microns. Warm dust, between minus 280 and 80 degrees Fahrenheit, can be traced at 24 micron wavelengths.
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 shows 24 micron emission due to 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."
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.
Interestingly, there is also 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 correct and the most massive disks form their planets first and the weaker disks take 10 to 100 times longer, then as many as 62 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 for evidence that terrestrial planets 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.
Source: University of Arizona