astronomy ,

Playing hide and seek with exoplanets


This illustration shows an Earth-like planet orbiting a nearby star. (Credit: NASA, ESA and G. Bacon/STScI)

U. COLORADO (US)—A precise “laser ruler” is being developed to look for Earth-like planets around other stars. The device will measure tiny changes in infrared light caused by the gravitational wobble of small, cool stars as they are tugged back and forth by their rocky planets.

Scientists at the University of Colorado at Boulder and the National Institute of Standards and Technology are working on the device, which uses technology based on 2005 Nobel Prize winning research conducted at JILA, a joint institute of the two.

The gravitational dance depends on the size of the star and the size of the planet and produces changes in the star’s radial velocity, explains Steve Osterman, research associate at CU-Boulder and principal investigator on the project.

A star’s radial velocity is the speed it is moving toward or away from Earth during such faint wobbles.

While astronomers have used the radial wobble of stars to detect several hundred planets outside our solar system, almost all have been giant, gaseous planets orbiting extremely close to their parent stars, says Osterman.

The new technology involves devices known as mode-locked lasers that deliver ultrashort pulses of infrared laser light less than a billionth of a second long, enabling a much more precise planet detection system, he explains.

Linked to an atomic clock, the laser ruler consists of thousands of closely spaced “tick-marks” representing successive infrared light frequencies that resemble the teeth of a comb, says NIST scientist Scott Diddams, who is collaborating with Osterman.

The comb makes it possible to measure minute changes in the light waves created by the motions of small, relatively cool M stars as they interact with planets by providing a precise calibration for spectrographs that analyze light coming from stars and planets.

The technique will allow the team to observe the stars in the near-infrared spectrum where they shine the brightest.

Osterman explains that the key to finding Earth-like planets is measuring the Doppler shift of the stars as they wobble during planet interactions. When a star is moving toward Earth, its wavelengths “bunch up” and shorten, and when the star is moving away from Earth, the wavelengths stretch out. By detecting extraordinarily faint wobbles, the researchers should be able to deduce the size of the planets and the distance of their orbit from the parent star.

“We have come up with a good ruler for measuring changes in the wobble of these small stars in the near-infrared wavelength of the spectrum,” says Osterman. “Since these M stars are much more common than larger stars, this gives us a lot more targets and should make it easier for us to detect rocky and perhaps even habitable planets.”

Osterman says M stars can be as small as one-tenth the mass and significantly older than Earth’s Sun. “We think our new calibration technology will make it as much as 10 to 20 times easier to detect habitable planets around these M stars,” he says.

Astronomers are particularly interested in the habitable zones of planets around other solar systems, zones marked by relatively moderate temperatures which have the potential to host liquid water. While at least one rocky planet slightly larger than Earth was recently identified by a French-led team, it orbits so close to the parent star that high temperatures and high radiation preclude the chances for life, Osterman says.

The Boulder researchers plan to take the new laser instrument to the Apache Point Observatory northeast of Las Cruces, N.M., in spring 2010 and integrate it with a new planet-finding instrument being developed at the University of Florida, says Osterman. “This will begin our search for Earth-like planets around these tiny stars.”

In addition to looking for Earth-like planets around low-mass stars, researchers say the comb technology will make it possible to peer through the dust clouds of young stellar systems more clearly, says John Bally of the Center for Astrophysics and Space Astronomy.

The technology may make it possible to learn more about the movements of massive, Jupiter-like planets in young planetary systems as they migrate toward their parent stars, he explains.

Other projects that will be made possible by the technology include studies of the atmospheres of young or cool stars as well as precise near-infrared observations of planetary atmospheres in our own solar system, according to the team.

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