Weirdest Planets
Strange New Worlds in our Universe

Weirdest Planets
Strange New Worlds in our Universe
An Earth analog (also referred to as a Twin Earth, Earth Twin, Second Earth, Alien Earth, Earth 2 or Earth-like planet) is a theoretical other planet (or world) with conditions similar to Earth.

Humans have long speculated on the existence of such a planet and there is considerable interest in the subject expressed in science, philosophy, science fiction and popular culture.

The probability of finding Earth-like planets which is also called an Earth analog depends mostly on the attributes which are expected to be similar, and these vary greatly.

Generally it is considered that it would be a terrestrial planet and there have been several scientific studies aimed at finding such planets.

Often implied but not limited to are such criteria as planet size, star size (i.e. simlar to the Sun), orbital distance and stability, axial tilt and rotation, similar geography, oceans, air and weather conditions, magnetic field and even the presence Earth-like complex life (possibly through convergent evolution or parallel evolution).

If there is complex life, there could be some forests covering much of the land. If there is intelligent life, some parts of land could be covered in cities. Some things that are assumed of such a planet may be unlikely due to Earth's own history.

For instance the Earth's atmosphere was not always oxygen-rich and this is a biosignature from the emergence of photosynthetic life. The formation, presence, influence on these characteristics of the moon (such as tidal forces) may also pose a problem in finding an Earth analog.

On 2 February 2011, the Kepler team announced the results of analysis of the data taken between 2 May and 16 September 2009.

They found 1235 planetary candidates circling 997 host stars. (The numbers that follow assume the candidates are really planets, though the official papers call them only candidates. Independent analysis indicates that at least 90% of them are real planets and not false positives.)

68 planets were approximately Earth-size, 288 super-Earth-size, 662 Neptune-size, 165 Jupiter-size, and 19 up to twice the size of Jupiter. 54 planets were within the habitable zone, including 5 less than twice the size of the Earth.

In contrast to previous work, roughly 74% of the planets are smaller than Neptune, most likely as a result of previous work finding large planets more easily than smaller ones.

The observed planet count versus size increases to a peak at two to three times Earth-size and then declines inversely proportional to area of the planet. The best estimate (as of March, 2011), after accounting for observational biases, was: 5.4% of stars host Earth-size candidates, 6.8% host super-Earth-size candidates, 19.3% host Neptune-size candidates, and 2.55% host Jupiter-size or larger candidates.

Multi-planet systems are common; 17% of the host stars have multi-candidate systems, and 33.9% of all the planets are in multiple planet systems.