One of the most popular topics in astronomy today is the hunt for extrasolar planets, or planets that revolve around a star other than the Sun, and its “holy grail” of finding an Earth-like planet. Such a discovery could come within four years if NASA’s planet-hunting Kepler spacecraft does its job as advertised. What many astronomers wonder, however, is what these “exo-Earths” will look like when found.
What Makes a Planet Earth-like?
A planet doesn't have to look like Earth to be considered Earth-like. Different astronomers have different definitions for what makes a planet Earth-like, often based on planet size or orbit rather than appearance. For a planet to actually resemble Earth, however, it would have to orbit within its star's habitable zone.
The habitable zone of a star refers to the region around a star in which it would be possible for any water on an orbiting planet to exist as a liquid, rather than as ice or vapor. For a star the size of the Sun, the habitable zone happens to be right around Earth’s orbit.
To find exo-Earths, NASA’s Kepler mission is looking for planets in roughly Earth-sized orbits around roughly Sun-sized stars. In its mission overview, NASA states that by the end of its 4-year mission, Kepler may have found as many as 50 such planets identical in size to Earth and over 600 planets up to two Earths in size.
What an Extrasolar Earth Might Look Like
It will be a long time before astronomers can guess what these new Earths might actually look like. When they do, however, some astronomers are predicting that these newly-discovered exo-Earths won’t look much like Earth at all.
"I think that's going to be the surprise," says Geoff Marcy, an astronomer at UC Berkeley who has found more exoplanets than anyone else, "I believe that rocky planets are going to be much more diverse, even bizarre, compared to our expectations."
At one extreme is the desert possibility. Just because the habitable zone means water can exist on the planet doesn’t mean the water will be there. Any water that is around when a planet forms tends to get burned away by stars when they are young and very hot. Water is often delivered to a planet’s surface later by ice-carrying meteorites.
Without these water deposits it won’t matter if a planet orbits in the habitable zone. The entire planet would be a dry desert (like Mars, which orbits outside of the Sun’s habitable zone and has all its water trapped as ice at the poles, leaving the rest of it a wasteland). Such a barren shell would be unlikely to support life.
At the other extreme is the possibility of an exo-Earth that holds large amounts of water. It is conceivable that a planet might be entirely covered by a global ocean, like the movie Waterworld brought to life (a local example is Europa, one of Jupiter’s moons, which astronomers believe may be covered by an ocean under a thick layer of ice).
A True Exo-Earth?
So might the first exo-Earth be a desert or an ocean? "I think we'll find all of those things," says Bill Borucki, the principal investigator for the Kepler mission. Between the two extremes, of course, lies that “just right” category that our Earth occupies. The great hope of astronomers is that they can find a “true” exo-Earth: an Earth-sized planet in an Earth-sized orbit around a Sun-like star, with a surface that has large amounts of water, but also land masses. Such a planet would be the first and best hope of someday finding familiar-looking life on other worlds.
What Makes a Planet Earth-like?
A planet doesn't have to look like Earth to be considered Earth-like. Different astronomers have different definitions for what makes a planet Earth-like, often based on planet size or orbit rather than appearance. For a planet to actually resemble Earth, however, it would have to orbit within its star's habitable zone.
The habitable zone of a star refers to the region around a star in which it would be possible for any water on an orbiting planet to exist as a liquid, rather than as ice or vapor. For a star the size of the Sun, the habitable zone happens to be right around Earth’s orbit.
To find exo-Earths, NASA’s Kepler mission is looking for planets in roughly Earth-sized orbits around roughly Sun-sized stars. In its mission overview, NASA states that by the end of its 4-year mission, Kepler may have found as many as 50 such planets identical in size to Earth and over 600 planets up to two Earths in size.
What an Extrasolar Earth Might Look Like
It will be a long time before astronomers can guess what these new Earths might actually look like. When they do, however, some astronomers are predicting that these newly-discovered exo-Earths won’t look much like Earth at all.
"I think that's going to be the surprise," says Geoff Marcy, an astronomer at UC Berkeley who has found more exoplanets than anyone else, "I believe that rocky planets are going to be much more diverse, even bizarre, compared to our expectations."
At one extreme is the desert possibility. Just because the habitable zone means water can exist on the planet doesn’t mean the water will be there. Any water that is around when a planet forms tends to get burned away by stars when they are young and very hot. Water is often delivered to a planet’s surface later by ice-carrying meteorites.
Without these water deposits it won’t matter if a planet orbits in the habitable zone. The entire planet would be a dry desert (like Mars, which orbits outside of the Sun’s habitable zone and has all its water trapped as ice at the poles, leaving the rest of it a wasteland). Such a barren shell would be unlikely to support life.
At the other extreme is the possibility of an exo-Earth that holds large amounts of water. It is conceivable that a planet might be entirely covered by a global ocean, like the movie Waterworld brought to life (a local example is Europa, one of Jupiter’s moons, which astronomers believe may be covered by an ocean under a thick layer of ice).
A True Exo-Earth?
So might the first exo-Earth be a desert or an ocean? "I think we'll find all of those things," says Bill Borucki, the principal investigator for the Kepler mission. Between the two extremes, of course, lies that “just right” category that our Earth occupies. The great hope of astronomers is that they can find a “true” exo-Earth: an Earth-sized planet in an Earth-sized orbit around a Sun-like star, with a surface that has large amounts of water, but also land masses. Such a planet would be the first and best hope of someday finding familiar-looking life on other worlds.
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