Does a twin Earth exist somewhere in our galaxy? NASA's Kepler spacecraft just launched to find such worlds, though in a very specific area. If that search succeeds, the next questions driving research will be: Is that planet habitable? Does it have an Earth-like atmosphere?
Astronomers say the James Webb Space Telescope (scheduled for launch in 2013) can help. In a new study, Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and Wesley Traub (Jet Propulsion Laboratory) examined the ability of JWST to characterize the atmospheres of hypothetical Earth-like planets during a transit, when part of the light of the star gets filtered through the planet's atmosphere. They found that JWST would be able to detect certain gases called biomarkers, such as ozone and methane, only for the closest Earth-size worlds.
In a transit event, a distant, extrasolar planet crosses in front of its star as seen from Earth. As the planet transits, gases in its atmosphere absorb a tiny fraction of the star's light, leaving fingerprints specific to each gas. By splitting the star's light into a rainbow of colors or spectrum, astronomers can look for those fingerprints. Kaltenegger and Traub studied whether those fingerprints would be detectable by JWST.
Artist's conception shows a hypothetical twin Earth orbiting a Sun-like star. A new study shows that characterizing a distant Earth's atmosphere will be difficult, even using next-generation technology like the James Webb Space Telescope. If an Earth-like world is nearby, though, then by adding observations of a number of transits, astronomers should be able to detect biomarkers like methane or ozone. Credit: David A. Aguilar (CfA)
"We'll have to be really lucky to decipher an Earth-like planet's atmosphere during a transit event so that we can tell it is Earth-like," said Kaltenegger. "We will need to add up many transits to do so - hundreds of them, even for stars as close as 20 light-years away."
The transit technique is very challenging. If Earth were the size of a basketball, the atmosphere would be as thin as a sheet of paper, so the resulting signal is incredibly tiny. Moreover, this method only works when the planet is in front of its star, and each transit lasts for a few hours at most.
Kaltenegger and Traub first considered an Earth-like world orbiting a Sun-like star. To get a detectable signal from a single transit, the star and planet would have to be extremely close to Earth. The only Sun-like star close enough is Alpha Centauri A. No such world has been found yet, but technology is only now becoming capable of detecting Earth-sized worlds.
The study also considered planets orbiting red dwarf stars. Such stars, called type M, are the most abundant in the Milky Way - far more common than yellow, type G stars like the Sun. They are also cooler and dimmer than the Sun, as well as smaller, which makes finding an Earth-like planet transiting an M star easier.
An Earth-like world would have to orbit close to a red dwarf to be warm enough for liquid water. As a result, the planet would orbit more quickly and each transit would last a couple of hours to mere minutes. But it would undergo more transits in a given amount of time. Astronomers could improve their chances of detecting the atmosphere by adding the signal from several transits, making red dwarf stars appealing targets because of their more frequent transits.
An Earth-like world orbiting a star like the Sun would undergo a 10-hour transit once every year. Accumulating 100 hours of transit observations would take 10 years. In contrast, an Earth orbiting a mid-sized red dwarf star would undergo a one-hour transit once every 10 days. Accumulating 100 hours of transit observations would take less than three years.
"Nearby red dwarf stars offer the best possibility of detecting biomarkers in a transiting Earth's atmosphere," said Kaltenegger.
"Ultimately, direct imaging - studying photons of light from the planet itself - may prove a more powerful method of characterizing the atmosphere of Earth-like worlds than the transit technique," said Traub.
Both NASA's Spitzer and Hubble Space Telescopes have studied the atmospheric compositions of extremely hot, gas-giant extrasolar planets. The characterization of a "pale blue dot" is the next step from there, whether by adding up hundreds of transits of one planet or by blocking out the starlight and analyzing the planet's light directly.
In a best-case scenario, Alpha Centauri A may turn out to have a transiting Earth-like planet that no one has spotted yet. Then, astronomers would need only a handful of transits to decipher that planet's atmosphere and possibly confirm the existence of the first twin Earth.
This research was partially funded by NASA.
Article: L. Kaltenegger, W.A. Traub, 'Transits of Earth-Like Planets', arXiv:0903.3371v1 [astro-ph.IM]
- PHYSICAL SCIENCES
- EARTH SCIENCES
- LIFE SCIENCES
- SOCIAL SCIENCES
Subscribe to the newsletter
Stay in touch with the scientific world!
Know Science And Want To Write?
- Part I: Bee Deaths Mystery Solved? Neonicotinoids (Neonics) May Actually Help Bee Health
- 3X Saturated Fat In The Diet Doesn't Increase It In Blood
- Eosinophilic Esophagitis: Genetic Clues Of Severe Food Allergy
- Still No Contact with NASA's Sun-watching Probe
- Is Religion A Consolation Worth Having?
- Why Computer Programs Can't Understand Truth - And Ethics Of Artificial Intelligence Babies
- Health Risks Of Nicotine Cast Doubt On 'Safer' E-cigarettes
- "As an earlier cigarette smoker and now e-cig smoker, I can confirm they don't help you quit. As..."
- "Reality? What is reality if you use irrational arguments to justify man's cruelty toward..."
- "Always loved the Heels that showed of the instep (arches) ..."
- "By the way, I am a fan of your blog. It's one of the few places I can follow physics without getting..."
- "Hello Anon,you're entirely right, it's arbitrary and it does not provide protection against cases..."
- Gene in kidney may play role in high blood pressure
- Panel-based genetic diagnostic testing for inherited eye disease proves highly accurate
- Research finds tooth enamel fast-track in humans
- Good news for cocaine users: Caffeine counters cocaine's effects on women's estrus cycles
- Clipping proteins that package genes may limit abnormal cell growth in tumors