Are there other planets like Earth?
Are there other planets like Earth?
One of the astonishing breakthroughs in observational astronomy of the 1990s was detecting the first planets outside our solar system. As of November 2006, astronomers
had found 210 extrasolar planets in 180 different planetary systems. The bulk of these planets are massive Jupiter-sized worlds. As of yet, technology does not allow astronomers to find distant earthlike planets around other stars. But that's not to say the search for such worlds hasn't begun.
Despite the fact that earthlike planets have not yet been found, astronomers feel there's good reason to think many exist.The history of extrasolar-planet discoveries around ordinary stars reaches back to 1995, when astronomers detected a planet circling the star 51 Pegasi.The planet is a gas giant more massive than Saturn that orbits its star every 4.2 days. The planet is so dose
to its star {Mercury orbits the Sun every 88 days), that astronomers termed it and many other massive, close-orbiting planets they began to find "hot Jupiters."
The discoveries of 51 Pegasi's planet and others like it puzzled astronomers, at first, because, as they understood solar system formation, gas giants formed on the outer fringes, not dose in to stars. How could hot Jupiters form so dose to their stars, when the star's heat would presumably destroy the planet's gases before they came together to make a planet? In addition to the puzzling results on large planets, astronomers found no smaller planets similar to Earth in their searches.
FAMILY PORTRAIT. In these six portraits taken by the outbound Voyager 1 (launched September 5,1977) spacecraft in 1990, six planets are visible. Starting at top left, they are: 1) Venus, 2) Earth, 3) Jupiter, 4) Saturn, 5) Uranus, and 6) Neptune, nasa
GOOD EC0BALANCE.
Earth affords life all the right luxuries — moderate temperatures, lush vegetation, and abundant water. This is a photo of the United States' Great Lakes region.
SEAWIFS PROJECT
Jupiters. It's becoming dear that hot Jupiters, once thought to be the norm, are actually the exception.
As the list of extrasolar planets grows, astronomers are discovering a lot about what it takes to host a planetary system. For example, more massive stars are more likely to have planets than lighter ones.
M dwarfs, which have masses V\o to Vs of the Sun's, seem to lack planets entirely. K- type stars, with masses of Vs to % the mass of the Sun, have only a small number of planets — about 3 to 4 percent of the time.
G-type stars like the Sun, thus far, show planets 7 percent of the time. The leaders, with 10 percent of their type bearing planets, are F stars — 30 to 50 percent more massive than the Sun.
Additionally, single stars (in the minority) are the best hosts for planetary systems.
Relatively few binary systems seem to have planets orbiting them. And the chemistry of stars matters, too: Stars rich in metals, elements heavier than hydrogen and helium, are better hosts for planetary systems than less evolved stars.
The search for extrasolar planets has sharpened astronomers' views of how solar systems form.The leading model, called
But, recently, the trend has reversed. With new technology and new planetsearching techniques, astronomers have found planets smaller than the hot
core accretion, suggests a rotating planetary disk settles around a young star, and then billions of collisions in the disk cause particles to clump together. As more and more particles stick together, protoplanets several miles across eventually form, and they become planetary cores.
The process happens relatively quickly in the inner part of a solar system, where rocky-planet building takes place. Terrestrial planets like those in our solar system could come together in as little as 100,000 years. But the creation of gas giants is not well explained in the core-accretion model. The gravitational-instability model is an alternative, less popular, idea. This says instabilities form as a disk collapses, leading to the clumping together of matter into planets. Neither idea yet addresses one surprise from the exoplanet-discovery program: the fact that nearly all exoplanets have orbits that are far more eccentric than those of the planets in our solar system.
Despite the mysteries, explanations of exoplanets are getting better by the month, as discoveries increase. The best places to find them are in the younger, metal-rich areas of spiral galaxies. Planets are most likely found in the galaxy's arms, and in the less chaotic regions of them.
The best stars to look at are those ofG and F spectral types. Perhaps the best tip for exoplanet searchers is to keep looking, in
2007, the European Space Agency will launch the Kepler mission. The onboard instruments will search more than 100,000 stars for transits of earthlike planets. In 2009, NASA's Space Interferometry Mission will look for earthlike planets around the nearest 100 stars. A proposed Terrestrial Planet Finder, which may or may not achieve funding, was slated for launch in 2014. It would detect light from earthlike planets within 45 light-years of home. EE
▲ BIG GLOW. Earth's nighttime city lights glow as recorded from space by a United States Department of Defense Meteorological Satellite. Could civilizations on planets surrounding other stars likewise be casting light into the darkness? It soon may be possible to detect such light originating on other worlds, nasa
T HEAVY SMOKER. Popocatepetl, or Popo, sends a plume of smoke skyward January 23, 2001, as imaged from the space shuttle. The Aztecs gave the mountain the name Popocatepetl, which means "smoking mountain." Other Earth-like planets may produce similar smoke plumes.
WWW.ASTRONOMY.COM 59
NASA
One of the astonishing breakthroughs in observational astronomy of the 1990s was detecting the first planets outside our solar system. As of November 2006, astronomers
had found 210 extrasolar planets in 180 different planetary systems. The bulk of these planets are massive Jupiter-sized worlds. As of yet, technology does not allow astronomers to find distant earthlike planets around other stars. But that's not to say the search for such worlds hasn't begun.
Despite the fact that earthlike planets have not yet been found, astronomers feel there's good reason to think many exist.The history of extrasolar-planet discoveries around ordinary stars reaches back to 1995, when astronomers detected a planet circling the star 51 Pegasi.The planet is a gas giant more massive than Saturn that orbits its star every 4.2 days. The planet is so dose
to its star {Mercury orbits the Sun every 88 days), that astronomers termed it and many other massive, close-orbiting planets they began to find "hot Jupiters."
The discoveries of 51 Pegasi's planet and others like it puzzled astronomers, at first, because, as they understood solar system formation, gas giants formed on the outer fringes, not dose in to stars. How could hot Jupiters form so dose to their stars, when the star's heat would presumably destroy the planet's gases before they came together to make a planet? In addition to the puzzling results on large planets, astronomers found no smaller planets similar to Earth in their searches.
FAMILY PORTRAIT. In these six portraits taken by the outbound Voyager 1 (launched September 5,1977) spacecraft in 1990, six planets are visible. Starting at top left, they are: 1) Venus, 2) Earth, 3) Jupiter, 4) Saturn, 5) Uranus, and 6) Neptune, nasa
GOOD EC0BALANCE.
Earth affords life all the right luxuries — moderate temperatures, lush vegetation, and abundant water. This is a photo of the United States' Great Lakes region.
SEAWIFS PROJECT
Jupiters. It's becoming dear that hot Jupiters, once thought to be the norm, are actually the exception.
As the list of extrasolar planets grows, astronomers are discovering a lot about what it takes to host a planetary system. For example, more massive stars are more likely to have planets than lighter ones.
M dwarfs, which have masses V\o to Vs of the Sun's, seem to lack planets entirely. K- type stars, with masses of Vs to % the mass of the Sun, have only a small number of planets — about 3 to 4 percent of the time.
G-type stars like the Sun, thus far, show planets 7 percent of the time. The leaders, with 10 percent of their type bearing planets, are F stars — 30 to 50 percent more massive than the Sun.
Additionally, single stars (in the minority) are the best hosts for planetary systems.
Relatively few binary systems seem to have planets orbiting them. And the chemistry of stars matters, too: Stars rich in metals, elements heavier than hydrogen and helium, are better hosts for planetary systems than less evolved stars.
The search for extrasolar planets has sharpened astronomers' views of how solar systems form.The leading model, called
But, recently, the trend has reversed. With new technology and new planetsearching techniques, astronomers have found planets smaller than the hot
core accretion, suggests a rotating planetary disk settles around a young star, and then billions of collisions in the disk cause particles to clump together. As more and more particles stick together, protoplanets several miles across eventually form, and they become planetary cores.
The process happens relatively quickly in the inner part of a solar system, where rocky-planet building takes place. Terrestrial planets like those in our solar system could come together in as little as 100,000 years. But the creation of gas giants is not well explained in the core-accretion model. The gravitational-instability model is an alternative, less popular, idea. This says instabilities form as a disk collapses, leading to the clumping together of matter into planets. Neither idea yet addresses one surprise from the exoplanet-discovery program: the fact that nearly all exoplanets have orbits that are far more eccentric than those of the planets in our solar system.
Despite the mysteries, explanations of exoplanets are getting better by the month, as discoveries increase. The best places to find them are in the younger, metal-rich areas of spiral galaxies. Planets are most likely found in the galaxy's arms, and in the less chaotic regions of them.
The best stars to look at are those ofG and F spectral types. Perhaps the best tip for exoplanet searchers is to keep looking, in
2007, the European Space Agency will launch the Kepler mission. The onboard instruments will search more than 100,000 stars for transits of earthlike planets. In 2009, NASA's Space Interferometry Mission will look for earthlike planets around the nearest 100 stars. A proposed Terrestrial Planet Finder, which may or may not achieve funding, was slated for launch in 2014. It would detect light from earthlike planets within 45 light-years of home. EE
▲ BIG GLOW. Earth's nighttime city lights glow as recorded from space by a United States Department of Defense Meteorological Satellite. Could civilizations on planets surrounding other stars likewise be casting light into the darkness? It soon may be possible to detect such light originating on other worlds, nasa
T HEAVY SMOKER. Popocatepetl, or Popo, sends a plume of smoke skyward January 23, 2001, as imaged from the space shuttle. The Aztecs gave the mountain the name Popocatepetl, which means "smoking mountain." Other Earth-like planets may produce similar smoke plumes.
WWW.ASTRONOMY.COM 59
NASA
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