Extrasolar Planets

E

Stars with Planets

Planets have been detected around stars ranging from red dwarfs with less than half the mass of the Sun up to giant stars six times the mass of the Sun. The types of planets found around a star likely depend on the amount and kind of material left over after the star forms. Disks around small stars have less gas than disks around larger stars. Stars that are rich in elements heavier than hydrogen may have different kinds of planets from stars with a smaller supply of heavier chemical elements.

Astronomers have recently found planets around binary stars—most stars in our galaxy are in such double-star systems. In examples known so far, a planet orbits around one star in the system. However, calculations show it is also possible for a planet to orbit around the common center of gravity of two stars orbiting close together.

Astronomers use the term “habitable zone” for the region of space around a star where a planet with the right conditions could support life as we know it. The atmosphere and temperatures would allow liquid water to exist, and radiation from the star would not be damaging to life. In our solar system, Earth orbits within the Sun’s current habitable zone, with Mars on the zone’s outer edge. The possible habitable zone for a star varies according its size and temperature. A small cool star would have a nearer habitable zone than a larger, hotter star.

VI

History of Extrasolar Planet Research

In the early 1900s, measurements of distance to other stars and galaxies changed traditional views of our solar system’s place in the universe. For the first time, astronomers found evidence that our solar system is not in the center of the galaxy and that our galaxy occupies no special place in the universe. Earth seems to have no special significance to the rest of the universe. This knowledge made it seem more likely that many other stars should have solar systems and that some of those solar systems might have Earth-like planets.

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In the 1940s astronomers detected a wobble in the movement of Barnard’s star, the star closest to the Sun after the Alpha Centauri triple star system. They suspected that a large planet might be causing the wobble, but decades of measurement show that the wobble is probably due to some other mechanism.

In 1983 observations from the United States Infrared Astronomy Satellite (IRAS) showed that the star Vega is surrounded by a disk of dust. Vega is a bluish star about 26 light-years from Earth (a light-year is the distance light travels in a year—9.5 trillion km or 5.9 trillion mi). It became the first star other than the Sun known to have a solar system, although further observations have shown no evidence that any planets orbit Vega.

In 1995 astronomers from Geneva Observatory in Switzerland used the Doppler technique to discover a planet with a mass comparable to that of Jupiter orbiting 51 Pegasi (see 51 Pegasi Solar System), a Sun-like star 50 light-years away. The planet (called 51 Pegasi B) orbits 51 Pegasi every four days at a distance of only about 8 million km (about 5 million mi). That distance is less than one-seventh of the distance between the Sun and Mercury, our solar system’s innermost planet. The planet 51 Pegasi B must be intensely hot.

In 1998 astronomers discovered disks of dust around the stars Fomalhaut and HR 4796 and took a closer look at the dust around Vega. None of these stars showed evidence of having a planet, but astronomers found evidence of comet-like objects orbiting Fomalhaut and Vega. Other 1998 dust disk discoveries were the first disk of dust observed around a binary (double) star and the first disk of dust observed around a very massive star. Using the Infrared Space Observatory, scientists looked for material around 84 nearby stars. They announced the results of their survey in 1999. The survey revealed that most young stars are surrounded by a dusty disk, while older stars are not.

Between 1996 and 2002, a team of astronomers led by Americans Geoffrey Marcy and Paul Butler used Doppler shift techniques to find more than 30 extrasolar planets. The solar systems these planets form include the Tau Boötis, Upsilon Andromedae, 16 Cygni B, 47 Ursa Majoris, 55 Cancri, and 70 Virginis solar systems. Some of the systems, such as 47 Ursa Majoris, may be much like our solar system, but others have planets with wildly eccentric orbits or have huge planets very close to the star.

In 2005 two teams of astronomers, one led by Drake Deming of NASA’s Goddard Space Flight Center and the other by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, announced that they had directly detected infrared light from two different extrasolar planets. Both of the planets are extremely hot and therefore emit enough infrared light to be detected even against the glare from their parent stars.

In 2006 astronomers with the OGLE project reported the discovery of the smallest planet yet known to orbit a star from a distance of 2 astronomical units (AUs), similar in distance to the region between Mars and Jupiter. (One AU is equal to the distance between the Sun and the Earth.) Other small planets that have been detected orbit their companion star from a distance of about 0.15 AU. The newly discovered planet is about 5.5 times as massive as Earth, making it smaller than Neptune. The physical makeup of the planet is thought to be similar to Uranus and Neptune. The scientists used a gravitational lensing event to detect the planet. Because detecting a small planet with this method is so rare and difficult, the astronomers calculated that such planets must be very common. This finding lends greater support to the core accretion theory of solar system formation.

As techniques and technology improve, astronomers should be able to find smaller planets in more distant orbits around other stars. The space telescopes COROT and Kepler are designed to detect planets about the size of Earth or smaller. Discovering how common such planets are should help scientists estimate the odds that life as we know may have evolved elsewhere in the universe.

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