Earth’s Twin Planets

Gliese 581 is a star of spectral type M3V (a red dwarf) about 20 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star to the Sun. Observations suggests that the star has a planetary system consisting of three planets, designated Gliese 581 b, c and e in order of discovery. Additional outer planets, which received the designations Gliese 581 d, f, and g have been proposed, but the evidence that led to the discovery claims has been shown to be the result of stellar activity mimicking the radial velocity variations due to orbiting planets.

Gliese 581

Gliese 581 has been the subject of a "huge amount of attention" in the quest to discover the first habitable extrasolar planet; first for c, and then d and g. Gliese 581 c, the first low-mass extrasolar planet found near a habitable zone, was discovered in April 2007. It has since been shown that under known terrestrial planet climate models, Gliese 581 c is likely to have a runaway greenhouse effect, and hence is probably too hot to be habitable, analogous to Venus. The proposed planets Gliese 581 d and Gliese 581 g also received attention as being located within the habitable zone, but their existence has subsequently been put into doubt by some authors.

On 27 November 2012, the European Space Agency announced a debris disk, with at least ten times as many comets as the Solar System. This put constraints on possible planets beyond 0.75 AU.

Gliese 581 is known at least from 1886, when it was included to Eduard Schönfeld's Southern Durchmusterung (SD) — the fourth part of the Bonner Durchmusterung. The corresponding designation is BD -7 4003.

Kepler-22b is an extrasolar planet orbiting G-type star Kepler-22b. It is located about 600 light years from Earth in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star.

Kepler-22b

The planet's first transit in front of its host star was observed on Kepler's third day of scientific operations, in 12 May 2009. The third transit was detected on 15 December 2010. Additional confirmation data was provided by the Spitzer Space Telescope and ground-based observations. On 5 December 2011, the confirmation of the existence of Kepler-22b was announced

The only parameters of the planet's orbit that are currently available are its period, which is about 290 days, and its inclination, which is approximately 90°, so that it transits the disk of its star as seen from Earth.

No information is available about the shape of the planet's orbit. Many extrasolar planets are known to move in highly elliptical orbits. It is only known that its average orbital distance is within its host star's habitable zone. If Kepler-22b has a highly elongated orbit it may well only spend a small fraction of its time within this habitable zone, which would cause extreme temperature differences on the planet and might make it inhospitable.

In order to obtain information about the shape of the planet's orbit, other methods of planetary detection, such as the radial velocity method, need to be used. While such methods have been performed on the planet after its discovery, they have not yet detected what the orbital eccentricity of the planet actually is, and have as of March 2012 only set an upper limit on the mass of the planet.