8. Exoplanets

Summary In the 1990s it became clear that planets orbiting other stars than the Sun exist. Due to their hard detectability, up to now these exoplanets were only observed within a small region of the Milky Way. While exoplanets similar to the Earth orbiting stars similar to the Sun in similar distances have been found, no extraterrestrial life has been discovered so far, even though its existence seems to be likely.

Keywords Extraterrestrial Life; Planets; Stars

Planet Discovery Neighbourhood in Milky Way Galaxy

Due to their hard detectability most observed exoplanets are located within only a small region of the Milky Way. (© NASA Jet Propulsion Laboratory / Wikimedia Commons / Public Domain)

The planets in our Solar System which orbit the Sun are not the only planets in the Universe. There are also planets orbiting other stars, which are called exoplanets or extrasolar planets. The first confirmed observations of exoplanets date to the 1990s. As of the beginning of 2019, the observations of about 4000 planets have been confirmed. However, due to their hard detectability most detected planets so far are located within only a small region of the Milky Way. It is likely that there exists at least one planet per star in the Milky Way, which would be at least 100 billion (1011) planets in our host galaxy. The masses of the detected planets range from twice the mass of the Moon – which corresponds to one fortieth of the mass of the Earth – to 30 times the mass of Jupiter – which corresponds to about 10,000 times the mass of the Earth.

The majority of exoplanets was found via indirect techniques. One such indirect planet detection technique consists in measuring the periodical change of a distant star’s speed towards Earth. This technique works as a planet does not actually orbit its host star, but actually both the planet and the star orbit their common center of mass. As the star’s mass is much larger than the planet’s mass, the center of mass is very close to the center of a star. However, the presence of a planet is still responsible for a slight wobbling of its host star which can be detected. Another indirect technique consists in measuring the periodical change of a star’s brightness which is due to a planet partially covering it when it passes through the line of sight between the star and the Earth. That slight periodical drop in brightness then depends on the cross-sectional area of the planet relative to the area of its star.

Artist’s impression of exoplanet orbiting two stars

An artist’s impression of an exoplanet which orbits a double-star system. In order to have a stable orbit in such a system, the planet has to have a distance from the two stars which is much larger than the inter-star distance. (© NASA, ESA, and G. Bacon (STScI) / Wikimedia Commons / CC-BY-4.0)

Both indirect exoplanet detection techniques can be easier realized for heavy and large planets which are close to their host stars. Indeed a relatively large fraction of detected observed exoplanets are large planets which are close to their host stars, so-called Hot Jupiters. As they could probably not be formed at that small distances from stars they probably migrated there after their formation. The frequent detection of Hot Jupiters does, however, not mean that Hot Jupiters are generally common in the Universe, due to the bias of the employed detection techniques. Also exoplanets orbiting double- or triple-star systems were found.

It is a thrilling research topic to find planets on which extraterrestrial life, that is life outside Earth which did not originate on Earth, did evolve or at least could have evolved. In that sense it is especially interesting to find planets which are similar to the Earth and orbit a living stable star like the Sun in the so-called habitable zone, that is the zone around a star where the temperature allows water on a planetary surface to be liquid. One example for an Earth-sized exoplanet in a habitable zone is Kepler-442b at a distance of about 1200 light-years from Earth whose detection was confirmed in 2015. It is a so-called super-Earth, which means that is is larger than Earth but smaller than the ice giants in our Solar System, and it is likely rocky instead of icy. One signature of extraterrestrial life could be the presence of oxygen in an exoplanet’s atmosphere, as in the case of Earth’s history atmospheric oxygen was produced by photosynthesizing microorganisms. However, no extraterrestrial life could be found up to now, and the search for it remains to be an active research topic.

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