Habitable Worlds around Distant Stars

An extrasolar planet is a planet that belongs to a star other than the Sun.

Modern astronomers use two general methods to detect extrasolar planets: direct—involving a search for telltale signs of a planet's infrared emissions—and indirect—involving precise observation of any perturbed motion (for example, wobbling) of the parent star or any periodic variation in the intensity or spectral properties of the parent star's light.

Growing evidence of planets around other stars is helping astronomers to validate the hypothesis that planet formation is a normal part of stellar evolution. Detailed physical evidence concerning extrasolar planets—especially if scientists can determine their frequency of occurrence as a function of the type of star—would greatly assist scientists in estimating the cosmic prevalence of life. If life originates on suitable (Earth-like) planets whenever it can (as many exobiologists currently hold), then knowing how abundant such suitable planets are in the Milky Way would allow scientists to make more credible decisions (that is, better educated guesses) about where to search for extraterrestrial intelligence and the probability of finding life (intelligent or otherwise) beyond humans' solar system.

Starting in the 1990s, scientists began to detect (through spectral light variations of the parent star) Jupiter-sized planets around Sunlike stars, such as 51 Pegasi, 70 Virginis, and 47 Ursae Majoris. Detailed computer analyses of spectrographic data have revealed that light from these stars appears redder and then bluer in an alternating periodic (sine wave) pattern. This periodic light pattern could indicate that the stars themselves are moving back and forth along the line of sight possibly due to a large (unseen) planetary object that is slightly pulling the stars away from (redder spectral data) or toward (bluer spectral data) Earth. The suspected planet around 51 Pegasi is sometimes referred to as a hot Jupiter, since it appears to be a large planet (about half of Jupiter's mass) that is located so

This is an artist's rendering of a large extrasolar planet, known as a hot Jupiter, orbiting around an alien star. This particular gas-giant planet orbits the yellow Sunlike star HD 209458, which is 150 light-years away from Earth. Astronomers used NASA's Hubble Space Telescope (HST) to look at this world and to make the first detection of an atmosphere around an extrasolar planet. The planet was not directly seen by the HST. Instead, the presence of sodium was detected in light that was filtered through the planet's atmosphere when it passed in front of its parent star, as seen from Earth—an event called a transit. The planet was discovered in 1999 by its subtle gravitational pull on the star. The planet has about 70 percent the mass of Jupiter and orbits the star at a distance of only 4 million miles (6.4 million km). (NASA and Greg Bacon [STScl/AVL])

close to its parent star that it completes an orbit in just a few days (approximately 4.23 days). The suspected planetary body orbiting 70 Virginis lies about one-half an astronomical unit (AU) distance from the star and has a mass approximately eight times that of Jupiter. Finally, the object orbiting 47 Ursae Majoris has an estimated mass that is 3.5 times that of Jupiter. It orbits the parent star at approximately AUs distance, taking about three years to complete one revolution.

To find extrasolar planets and characterize their atmospheres, scientists will use current (or planned) space-based observatories like the Spitzer Space Telescope (SST), the James Webb Space Telescope (JWST), and the Kepler spacecraft. In late 2003, NASA's Spitzer Space Telescope captured a dazzling image of a massive disc of dusty debris encircling a nearby star called Fomalhaut. Planetary scientists consider such discs as remnants of planetary construction and believe that Earth formed out of a similar disc.

The Spitzer Space Telescope is now helping scientists identify other stellar dust clouds that might mark the sites of developing planets. In 2004, this space-based infrared telescope gathered data to indicate a possible planet spinning its way through a clearing in a nearby star's dusty, planet-forming disc. Spitzer detected this clearing around the star CoKu Tau 4. Astronomers believe than an orbiting massive body, like a planet, may have swept away the star's disc material, leaving a central hole. The possible planet is theorized to be at least as massive as Jupiter and may have a similar appearance to what the giant planets in this (humans') solar system looked like billions of years ago. As shown in the artist's rendering, a graceful set of rings, much like Saturn's, spin high above the planet's cloudy atmosphere. The set of rings is formed from countless small orbiting particles of dust and ice—leftovers from the initial gravitational collapse that formed the suspected giant planet.

This is an artist's rendering of a suspected extrasolar planet. In May 2004, a clearing was detected around the star CoKu Tau 4 by NASA's Spitzer Space Telescope. Astronomers believe that an orbiting massive body (like the planet depicted here) may have swept away the star's disc material, leaving a central hole. (NASA/JPL)

If some day human beings were able to visit an extrasolar planet like this one, they would have a very different view of the universe. The sky, instead of being the familiar dark expanse lit by distant stars, would be dominated by the thick disc of dust that fills the young planetary system. The view looking toward the alien solar system's parent star (CoKu Tau 4) would be relatively clear because the dust in the interior disk has already fallen into the accreting star. A bright band would appear to surround the central star, caused by the central star's light being scattered back by the dust in the disc. Looking away from CoKu Tau 4, the dusty disc would appear dark, blotting out light from all the stars in the sky except those that lie well above the plane of the disc.

The James Webb Space Telescope (JWST) will be a large single telescope that is folded to fit inside its launch vehicle and cooled to low temperatures in deep space to enhance its sensitivity to faint, distant objects. Mission controllers will operate JWST in an orbit far from Earth, away from the thermal energy (heat) radiated to space by humans' home planet. It is scheduled for launch in 2011, and one of this observatory's main science goals is to determine how planetary systems form and interact. The JWST will be able to observe evidence of the formation of planetary systems (some of which may be similar to this solar system) by mapping the light from the clouds of dust grains orbiting stars.

The star, Beta Pictoris, has such a cloud, as was discovered by the Infrared Astronomical Satellite (IRAS). These clouds are bright near the host stars and may be divided into rings by the gravitational influence of large planets. Scientists speculate that this dust represents material forming into planets. Around older stars, such dust clouds may be the debris of material that failed to condense into planets. The JWST will have unprecedented sensitivity to observe faint dust clouds around nearby stars. The infrared wavelength range is also the best way to search for planets directly because they are brighter, relative to their central stars. For example, at visible wavelengths, Jupiter is about 100 million times fainter than the Sun, but in the infrared, it is only 10,000 times fainter. A planet like Jupiter would be difficult to observe directly with any telescope on Earth, but the JWST has a chance—because it operates in space beyond the disturbing influences of the terrestrial atmosphere.

Currently scheduled for launch in November 2008, NASA's Kepler spacecraft will use a unique space-based telescope that has been specifically designed to search for Earth-like planets around stars beyond the solar system. The Kepler spacecraft will allow scientists to search the Milky Way galaxy for Earth-size or even smaller planets. The extrasolar planets discovered thus far are giant planets, similar to Jupiter, and are probably composed mostly of hydrogen and helium. So exobiologists believe that such Jupiter-sized planets are unlikely to harbor life. However, some

This artist's rendering shows the view from a hypothetical moon in orbit around the first known planet to reside in a tight-knit triple star system. The planet, called HD 188753 Ab, is a gas giant with about 1.14 times the mass of Jupiter and an orbital period of 3.3 days. The giant planet travels around a single star that is orbited by a pair of pirouetting stars. The planet with three suns was discovered using the Keck I telescope atop Mauna Kea in Hawaii. The triple star family is called HD 188753 and is located 149 light-years away in the constellation Cygnus. (NASA/JPL-Caltech)

This artist's rendering shows the view from a hypothetical moon in orbit around the first known planet to reside in a tight-knit triple star system. The planet, called HD 188753 Ab, is a gas giant with about 1.14 times the mass of Jupiter and an orbital period of 3.3 days. The giant planet travels around a single star that is orbited by a pair of pirouetting stars. The planet with three suns was discovered using the Keck I telescope atop Mauna Kea in Hawaii. The triple star family is called HD 188753 and is located 149 light-years away in the constellation Cygnus. (NASA/JPL-Caltech)

scientists have suggested that these massive planets may have moons with an atmosphere and liquid water on the surface. In such cases, life could arise in these planetary systems. The Kepler mission is especially important because none of the extrasolar planet detection methods used to date have had the capability of finding Earth-sized planets—that is, planets that are 30 to 600 times less massive than Jupiter. Furthermore, none of the giant extrasolar planets discovered to date probably has not had liquid water on its surface or even a solid surface.

The Kepler spacecraft is different from previous ways of looking for planets because it will look for the transit signature of planets. A transit occurs each time a planet crosses the line-of-sight between the planet's parent star that it is orbiting and the observer. When this happens, the planet blocks some of the light from its star, resulting in a periodic dimming. This periodic signature is used to detect the planet and to determine its size and its orbit. Three transits of a star, all with a consistent period, brightness change, and duration, provide a robust method of

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