Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars. At one time there were probably many more terrestrials, but most have been ejected from the solar system or otherwise destroyed. Only one terrestrial planet, Earth, is known to have an active hydrosphere.

Extrasolar terrestrial planets

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 inturruptions 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 terrestial. 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.

See also

• Jovian planet/Gas giant planet
• Chthonian planet
• Planetary habitability

References

• 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.

Planets | Planets of the solar system

Tē-kiû-hêng he̍k-chheⁿ | Земеподобна планета | Planeta tel·lúric | Terestrická planeta | Erdähnlicher Planet | Planète tellurique | Terestrički planet | Pianeta terrestre | Aardse planeet | 地球型惑星 | Planeta telúrico | Планеты земной группы | Terestriálna planéta | Zemeljski planet | Maankaltainen planeetta | Yerbenzeri gezegen | 类地行星