Most extrasolar planets discovered so far are "hot Jupiters," gas giants orbiting close to their parent stars. But according to current theory, such planets should form much farther from their stars, where temperatures are cold enough for water and other volatiles to freeze. Finding these planets close to their stars is like finding snowmen close to a furnace. You might wonder how they got there.
Image right: Artist's concept of protoplanetary disc and SIM PlanetQuest. Image credit: NASA/JPL
How to find the answer? The same way one learns how Anakin Skywalker became Darth Vader: Look to the early years.
Dr. Charles Beichman and his team will use NASA's SIM PlanetQuest, a very precise space telescope scheduled for launch in 2011, to observe young stars on the verge of igniting the nuclear fusion that powers what astronomers call the "main sequence" of a star's life.
"We don't know anything about planets around very young stars because the radial velocity method doesn't work with them," Beichman said. Radial velocity is a technique for observing how a star wobbles in response to the gravitational pull of orbiting planets and other objects. It has been used to detect most of the roughly 150 known planets beyond our solar system. The method works well for very large objects in close orbit around their stars, but not so well for smaller or more distant orbiting companions. And it can't get good readings on very young stars, whose active surfaces confound the observations. (More about the wobble)
The Kids Next Door
But with SIM PlanetQuest, scientists will be able to measure a star's wobble with exquisite precision against a background of stars so distant they appear to be stationary. Using this technique, Beichman's group expects to be able to detect planets of less than one Jupiter mass at distances greater than five times the distance from Earth to the Sun, which is where Jupiter orbits in our own solar system, as well as larger planets closer in.
Image left: Radial velocity measures the red and blue shift caused by a star's 'wobble' as it is tugged back and forth by its planetary companions. Image credit: NASA/JPL
Beichman plans to search for planets around 150 of the nearest and youngest sunlike stars. "Nearest" in this case means within 489 light-years, and "youngest" means from half a million to 100 million years old. In astronomical terms, these are the kids next door. And a wild and reckless youth they may be having!
If, as theory suggests, giant planets form in distant orbits and migrate inward to where they are often found in mature systems, then something is robbing the young planets of the momentum that would otherwise keep them in their initial orbits. It could be drag from plowing through the dusty disc from which the planets formed. Or it could be wrestling matches with other giant planets, in which they gravitationally grab and hurl each other into new orbits. In that case, for every planet thrown closer to the parent star (possibly falling into it), a planet would be kicked further out (possibly leaving the planetary system entirely).
One model suggests that for every Jupiter-size planet that survives in mature systems, three may have formed initially. That would mean that two out of three young Jupiters are destroyed or ejected from their planetary systems. Current observations show that many extrasolar planets are in highly elliptical orbits, which increase the chances of interactions with other planets.
A demolition derby like this doesn't bode well for the development of Earthlike planets in these systems. "It's like an 18-wheeler truck careening around a parking lot, and you're in a VW huddling under the dash, hoping not to get hit," Beichman said. "It disrupts any chance you would have of an 'Earth' finding a stable orbit."
SIM PlanetQuest's ability to determine the distances to relatively nearby stars, combined with observations of the stars' effects on their binary partners or the discs of dust and gas that surround them, will enable Beichman's group to calculate the masses of about 100 young stars with unprecedented accuracy. Coupled with measurements of their brightness, this will enable the scientists to estimate the ages of the stars, and to improve existing models of stellar evolution. "One of the uglier secrets of astronomy," Beichman confided, "is that nobody knows anything about ages of stars. Stars do not come with sell-by dates."
Armed with these improved models, Beichman's team will analyze the wobbles of a different group of about 150 stars to determine how many planets are in orbit around each one, how massive each planet is, and how far from the parent star it orbits. How can they tease out so much information from nearly imperceptible motions? Beichman draws a musical analogy. "Astronomers from Pythagoras to Kepler spoke of the music of the spheres," he said, "and they weren't far off."
Each planet induces a distinctive wobble in its parent star, determined by the planet's mass (relative to the star's mass) and its distance from the star. Multiple planets create multiple wobbles, superimposed on each other like the notes in a musical chord.
Just as a person with perfect pitch can identify the individual notes of a chord, scientists can identify the individual wobbles and interpret their meaning. "SIM," said Beichman, "has perfect pitch." And that is how we will understand the story young stars have to tell about the planets in their keeping -- the story before the story of mature extrasolar systems.
For more information about the search for new planets, visit NASA's PlanetQuest Web site.