Nature of Exoplanets 26 October 2016

Exoplanet Census as of 1 October 2016, there have been 3,532 exoplanets in 2,649 planetary systems and 595 multiple planetary systems confirmed. Kepler space telescope (since 2009) has found more than two thousand. Kepler has also detected a few thousand candidate planets, of which about 11% may be false positives.

Counts of Exoplanets

Jupiter vs Tres-3

Earth-Like Planets About 1 in 5 Sun-like stars have an "Earth-sized” planet in the habitable zone Assuming there are 200 billion stars in the Milky Way, one can hypothesize that there are 11 billion potentially habitable Earth-sized planets in the Milky Way

Remember The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life

Formation and evolution Planets form within a few tens of millions of years of their star forming, and there are stars that are forming today and other stars that are ten billion years old, so unlike the planets of the Solar System, which can only be observed as they are today, studying exoplanets allows the observation of exoplanets at different stages of evolution. When planets form they have hydrogen envelopes that cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen is eventually lost to space. This means that even terrestrial planets can start off with large radii. An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth

Atmospheres Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001. KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet. The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[99] In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14×106 km (9×106 mi) long

Exoplanet Atmospheres Several planets outside the Solar System have been observed to have atmospheres. At the present time, all atmosphere detections are of hot Jupiters or hot Neptunes that orbit very close to their star and thus have heated and extended atmospheres. Observations of exoplanet atmospheres are of two types. First, transmission photometry or spectra detect the light that passes through a planet's atmosphere as it transits in front of its star. Second, the direct emission from a planet atmosphere may be detected by differencing the star plus planet light obtained during most of the planet's orbit with the light of just the star during secondary eclipse (when the exoplanet is behind its star)

Atmospheric composition In 2001, sodium was detected in the atmosphere of HD 209458 b. In 2008, water, carbon monoxide, carbon dioxide and methane were detected in the atmosphere of HD 189733 b. In 2013, water was detected in the atmospheres of HD 209458 b, XO-1b, WASP-12b, WASP-17b, and WASP-19b. In July 2014, NASA announced finding very dry atmospheres on three exoplanets (HD 189733b, HD 209458b, WASP-12b) orbiting Sun-like stars In September 2014, NASA reported that HAT-P-11b is the first Neptune-sized exoplanet known to have a relatively cloud-free atmosphere and, as well, the first time molecules of any kind have been found, specifically water vapor, on such a relatively small exoplanet The presence of oxygen may be detectable by ground-based telescopes, which, if discovered, would suggest the presence of photosynthetic life on an exoplanet.

Summary Thousands of exoplanets, with many Kepler candidates still to be confirmed Hundreds are in multiple star systems About half have eccentric orbits, unlike our Solar System About 1 in 5 sunlike stars have an Earth-sized planet in the Habitable Zone