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Written by Adam Hadhazy
Media contact: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
Whitney.clavin@jpl.nasa.gov
Feature: 2011-110 April 7, 2011
NASA Telescope Ferrets Out Planet-Hunting Targets
The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-110&cid=release_2011-110
Astronomers have come up with a new way of identifying close, faint stars with NASA's Galaxy
Evolution Explorer satellite. The technique should help in the hunt for planets that lie beyond our
solar system, because nearby, hard-to-see stars could very well be home to the easiest-to-see alien
planets.
The glare of bright, shining stars has frustrated most efforts at visualizing distant worlds. So far,
only a handful of distant planets, or exoplanets, have been directly imaged. Small, newborn stars
are less blinding, making the planets easier to see, but the fact that these stars are dim means they
are hard to find in the first place. Fortunately, the young stars emit more ultraviolet light than
their older counterparts, which makes them conspicuous to the ultraviolet-detecting Galaxy
Evolution Explorer.
"We've discovered a new technique of using ultraviolet light to search for young, low-mass stars
near the Earth," said David Rodriguez, a graduate student of astronomy at UCLA, and lead
author of a recent study. "These young stars make excellent targets for future direct imaging of
exoplanets."
Tantrum-Throwing Baby Stars
Young stars, like human children, tend to be a bit unruly -- they spout a greater proportion of
energetic X-rays and ultraviolet light than more mature stars. In some cases, X-ray surveys can
pick out these youngsters due to the "racket" they cause. However, many smaller, less "noisy"
baby stars perfect for exoplanet imaging studies have gone undetected except in the most
detailed X-ray surveys. To date, such surveys have covered only a small percentage of the sky.
Rodriguez and his team figured the Galaxy Evolution Explorer, which has scanned about three-
quarters of the sky in ultraviolet light, could fill this gap. Astronomers compared readings from
the telescope with optical and infrared data to look for the telltale signature of rambunctious
junior stars. Follow-up observations of 24 candidates identified in this manner determined that
17 of the stars showed clear signs of youth, validating the team's approach.
"The Galaxy Evolution Explorer can readily select young, low-mass stars that are too faint to turn
up in all-sky X-ray surveys, which makes the telescope an incredibly useful tool," Rodriguez said.
Cool, Red and in the Neighborhood
Astronomers call the low-mass stars in question "M-class" stars. Also known as red dwarfs, these
stars glow a relatively cool crimson color compared to the hotter oranges and yellows of stars like
our sun, and the whites and blues of the most scorching stars. With data from the Galaxy
Evolution Explorer, astronomers could reap a bounty of these red dwarfs still in their cosmic
youth, under 100 million years old.
In many ways, these stars represent a best-case scenario for the direct imaging of exoplanets.
They are close and in clear lines-of-sight, which generally makes viewing easier. Their low mass
means they are dimmer than heavier stars, so their light is less likely to mask the feeble light of a
planet. And because they are young stars, their planets are freshly formed, and thus warmer and
brighter than older planetary bodies.
The Better to See Planets With
So far, only a handful of the more than 500 exoplanets on record have actually been "seen" by
our ground- and space-based telescopes. The vast majority of foreign worlds have instead turned
up via indirect means. One common technique, for instance, relies on detecting the slight
gravitational "wobbles" exoplanets impart to their host stars. Another technique, the "transit
method," registers the tiny dip in a star's light as an exoplanet crosses in front of it relative to our
vantage point. NASA's Kepler mission, in just its first four months of operations, has already
come up with a list of more than 1,200 candidate exoplanets using the transit method.
At a very basic level, directly imaging an exoplanet is worthwhile because, after all, "seeing is
believing," Rodriguez said. But catching a glimpse of an exoplanet also opens up novel scientific
avenues.
Direct imaging is well suited for seeing big planets circling host stars at considerable distances,
comparable to Uranus and Neptune in our solar system. Observing such arrangements is useful
for testing concepts of solar system evolution, Rodriguez said. Plus, gleaning details about the
atmospheres of imaged exoplanets is less difficult than indirectly investigating worlds that transit
their stars.
As for actually imaging clouds or surface features of exoplanets, however, that will have to wait.
Current images of exoplanets, while full of information, resemble fuzzy dots. But as technology
advances, ever more information about our close-by planetary brethren will emerge.
Data from NASA's Wide-field Infrared Survey Explorer (WISE) mission could also reveal stars
that would make good candidates for imaging planets. Its all-sky maps will allow scientists to
pick out nearby, young stars surrounded by warm disks of planetary debris that glow with
infrared light. Such stars are similar to the ones where planets have already been successfully
imaged.
The new study was published in the February issue of The Astrophysical Journal and includes co-
authors Mike Bessell (Australian National University), Ben Zuckerman (UCLA), and Joel
Kastner (Rochester Institute of Technology).
The California Institute of Technology in Pasadena leads the Galaxy Evolution Explorer mission
and is responsible for science operations and data analysis. NASA's Jet Propulsion Laboratory,
also in Pasadena, manages the mission and built the science instrument. Caltech manages JPL for
NASA. The mission was developed under NASA's Explorers Program managed by NASA's
Goddard Space Flight Center, Greenbelt, Md. Researchers sponsored by Yonsei University in
South Korea and the Centre National d'Etudes Spatiales (CNES) in France collaborated on this
mission.
Additional information about the Galaxy Evolution Explorer are online at
http://www.galex.caltech.edu and http://www.nasa.gov/galex .
More information on NASA's planet-hunting efforts is online at http://planetquest.jpl.nasa.gov .
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