Terrestrial planet
A
terrestrial planet or
telluric planet is a
planet which is primarily composed of
silicate rocks. The term is derived from the
Latin word for
Earth,
"Terra", so an alternate definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from
gas giants, which may not have solid surfaces and are composed mostly of some combination of
hydrogen,
helium, and
water existing in various
physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly
iron, with a surrounding silicate
mantle. The
Moon is similar, but lacks an iron core. Terrestrial planets have
canyons,
craters,
mountains, and
volcanoes. Terrestrial planets possess
secondary atmospheres; atmospheres generated through internal vulcanism or comet impacts, as opposed to the gas giants, which possess
primary atmospheres, atmospheres captured directly from the original
solar nebula.
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A view on Planet Earth (painting) |
Earth's
solar system has four terrestrial planets:
Mercury,
Venus,
Earth and
Mars. During the formation of the solar system, there were probably many more (
planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the
solar nebula. Only one terrestrial planet, Earth, is known to have an active
hydrosphere.
The majority of planets found outside our
solar system to date have been
gas giants, simply because gas giants are larger and therefore easier to see or infer from observation. However, a number of extrasolar planets are known or suspected to be terrestrial.
The first terrestrial
planets ever detected outside our solar system were detected by
Aleksander Wolszczan orbiting the
pulsar PSR B1257+12. Their masses being 0.02, 4.3, and 3.9
Earth masses. The planets were observed because their transit caused interruptions in the pulsar's radio emissions. Had they not been orbiting around a pulsar, they would never have been found.
When
51 Pegasi b, the first extrasolar planet found around a
fusing star, was discovered, many astronomers assumed it must be a gigantic terrestrial, as it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. However, subsequent measurements of its diameter confirmed it was a gas giant.
In June
2005, the first planet around a fusing star that is almost certainly terrestrial was found orbiting around the
red dwarf star Gliese 876, 15 light years away. That planet has a mass between six and nine times that of earth and an orbital period of just two Earth days.
On 10 August, 2005, the international team
Optical Gravitational Lensing Experiment spotted the signature of a cold planet designated
OGLE-2005-BLG-390Lb, about 5.5 times the mass of Earth, orbiting a star about 21,000
light years away in the constellation Scorpius. The planet revealed its existence through a technique known as
gravitational microlensing, currently unique in its capability to detect cool planets with masses down to that of Earth.
In late 2005, the same team discovered the planet
OGLE-2005-BLG-169Lb, which is 13 times the mass of Earth and orbiting a star approximately 9,000 light years away. This planet may be either a gas giant or terrestrial. The newly discovered planet orbits its parent star at a distance similar to that of our solar system's
asteroid belt.
Theoretically, there are two types of terrestrial or rocky planets, one dominated by silicon compounds, as Earth is, and another dominated by carbon compounds, like
carbonaceous chondrite asteroids. These are the
silicate planets and
carbon planets (or "diamond planets") respectively.
A number of telescopes capable of directly imaging extrasolar terrestrial planets are on the drawing board. These include the
Terrestrial Planet Finder,
Darwin (ESA),
New Worlds Imager, and
Overwhelmingly Large Telescope.
*
Jovian planet/
Gas giant planet
*
Chthonian planet*
Planetary habitability*
Found: one Earth-like planet Astronomers use gravity lensing to spot homely planets. By Mark Peplow, News @ Nature.com, 25 January 2006.
*
Beaulieu J.P., et al. (2006) Nature, 439, 437-440.
*
National Science Foundation press release "Closer to Home."
*
A New Path to New Earths National Science Foundation webcast.
*
Ogling Distant Stars National Science Foundation grant report.
*
Wolszczan's Pulsar Planets.
*
PLANET Homepage.
*
RoboNet Homepage.
*
OGLE Homepage.
*
MOA Homepage.
