The Planetary Horizon

UH Manoa Researchers Look to the Horizon of Future Planet Searches

A recent NASA study calculated the diameters of various types of planets given certain compositions and masses. There are many models predicting the potential sizes and locations for Earth-like planets, but because they are so small, they are difficult to find using current planet-hunting techniques.
Credit: Marc Kuchner/NASA GSFC.

In a paper published in the journal Science, three University of Hawaii at Manoa researchers and their colleagues review the prospects for discovering smaller planets more like Earth, some of which may even have conditions suitable for life. Astronomers reported the first planet around another Sun-like star in 1995 and since then have found more than 200 such planets, all thought to be "gas giants" made mostly of hydrogen and helium like Jupiter and Saturn in our Solar System.

"The most successful technique for discovering planets to date spreads light from the host star into its constitutive wavelengths (colors)" said lead author Eric Gaidos, who is an associate professor in the Department of Geology & Geophysics and the NASA Astrobiology Institute at UH Manoa. "A shift in wavelength, analogous to the change in pitch of the horn of a passing automobile, reveals any motion of the star along the line of sight. Monitoring of a star can detect periodic motion caused by the gravitational pull of any unseen, orbiting planet."

Improved techniques and the ability to monitor fainter stars now enable astronomers to discover smaller planets, particularly planets orbiting much closer to their host star than the Earth is to the Sun. New computer simulations such as those performed by Sean Raymond, co-author of the paper and NASA Postdoctoral Fellow at the University of Colorado Boulder, show how such planets could form further out and then "migrate" inwards to such orbits.

This image shows how the brightness of a parent star dims as a planet transits. This dimming of light can be seen by telescopes on Earth and is how many extrasolar planets have been discovered. However, this method works best when the planets are very large. Small, Earth-like planets may not dim their parent star’s light enough for them to be detected.
Credit: ESO

Another method now used to find planets is to observe the slight decrease in light from the star as an orbiting planet passes in front of it. This happens only for those planets whose orbits by chance are seen edge-on. Jupiter-sized planets can be found this way using telescopes on the ground, but Earth-size planets might be detected by the European CoRoT spacecraft, now in orbit, and NASA’s Kepler spacecraft, scheduled to launch in 2009.

"These methods can sometimes be combined to estimate the density of the planet, which will tell us whether the planet is composed mostly of rock and metal, like Earth, or something else such as water ice," said Gaidos.

Computational simulations by co-author Nader Haghighipour, a planetary dynamicist at the Institute for Astronomy and the Astrobiology Institute at UH Manoa, have shown that smaller Earth-sized planets can indeed exist in such tight planetary environments.

According to the paper, planets orbiting much closer to a star like the Sun will be much hotter and, like Mercury and Venus in our Solar System, inhospitable to life. However, many stars are much less bright than the Sun, and planets close to them could still orbit within a "habitable zone" where surface temperatures could permit stable liquid water.

"Explaining the formation of habitable planets in such environments is a challenging task. However, our simulations have been successful in determining condition under which planets similar to Earth can form in the habitable zones of less bright stars," said Haghighipour.

This artist’s illustration shows a system with a star much like our Sun and a gas-giant planet similar to Jupiter. Most of the more than 200 extrasolar planets known today are gas giants up to 4,000 times larger than Earth, and are thought to be uninhabitable for life as we know it.
Credit: NASA/JPL/CalTech/T.Pyle, SSC

Future space observatories beginning with NASA’s James Webb Space Telescope have the potential to study such planets and determine whether they have atmospheres or oceans.

Added Gaidos, "The discovery of another life-bearing planet would be a scientific triumph for humanity, but the study of many lifeless, un-Earthly worlds would nevertheless tell us about how planets form, and help us appreciate the Earth all that much more". Finding a habitable planet beyond our Solar System would vastly increase our understanding of how life is able to establish itself on distant worlds. As of now, the only example we have for how life works is on our own planet, and discovering a second location for life in the Universe is a primary goal of astrobiology.

Other researchers contributing to the paper were John Rayner of the Institute for Astronomy at UH Manoa, Eric Agol of the University of Washington and David Latham of the Harvard-Smithsonian Center for Astrophysics.



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