Exoplanets

Introduction

An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmation of an exoplanet detection occurred in 1992. As of 1 October 2019, there are 4,118 confirmed exoplanets in 3,063 systems, with 669 systems having more than one planet.

Drawing with 4 smallest planets of Kepler-90 larger than smallest planets in solar system, 4 largest planets similar size.
Figure 1. Artist's conception of exoplanets in Kepler-90 system compared to solar system.

Poster showing fictional party goers on exoplanet.
Figure 2. A NASA travel poster "spoof." Discovered in October 2013 using direct imaging, PSO J318.5-22 belongs to a special class of planets called rogue, or free-floating, planets. Wandering alone in the galaxy, they do not orbit a parent star. Not much is known about how these planets come to exist, but scientists theorize that they may be either failed stars or planets ejected from very young systems after an encounter with another planet. These rogue planets glow faintly from the heat of their formation. Once they cool down, they will be dancing in the dark.

 

There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected as of 2019 are inside the tidal locking zone. In several cases, multiple planets have been observed around a star. 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, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.

There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist. 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.

Besides exoplanets, there are also rogue planets, which do not orbit any star. These tend to be considered as a separate category, especially if they are gas giants, in which case they are often counted as sub-brown dwarfs, like WISE 0855−0714. The rogue planets in the Milky Way possibly number in the billions (or more). [Source: Wikipedia https://en.wikipedia.org/wiki/Exoplanet]

Poster showing fictional tourists on an exoplanet looking out of a vehicle window at a city.
Figure 3. Another NASA travel poster "spoof." Some 40 light-years from Earth, a planet called TRAPPIST-1e offers a heart-stopping view: brilliant objects in a red sky, looming like larger and smaller versions of our own moon. But these are no moons. They are other Earth-sized planets in a spectacular planetary system outside our own. These seven rocky worlds huddle around their small, dim, red star, like a family around a campfire. Any of them could harbor liquid water, but the planet shown here, fourth from the TRAPPIST-1 star, is in the habitable zone, the area around the star where liquid water is most likely to be detected. This system was revealed by the TRAnsiting Planets and PlanetIsmals Small Telescope (TRAPPIST) and NASA's Spitzer Space Telescope.

Resources

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Science Buddies Original
Career Profile
Astronomers think big! They want to understand the entire universe—the nature of the Sun, Moon, planets, stars, galaxies, and everything in between. An astronomer's work can be pure science—gathering and analyzing data from instruments and creating theories about the nature of cosmic objects—or the work can be applied to practical problems in space flight and navigation, or satellite communications. Read more
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News Article EurekAlert!
August 27, 2019
A new study by McGill University astronomers has found that the temperature on the nightsides of different hot Jupiters -- planets that are similar size in to Jupiter, but orbit other stars -- is surprisingly uniform, suggesting the dark sides of these massive gaseous planets have clouds made of minerals and rocks. Read more
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News Article EurekAlert!
September 26, 2019
Astronomers have discovered a giant Jupiter-like exoplanet in an unlikely location -- orbiting a small red dwarf star. Read more
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News Article EurekAlert!
September 27, 2019
An international team of researchers with participation from the University of Göttingen has discovered the first large gas giant orbiting a small star. The planet orbits the nearby red dwarf star GJ3512. This discovery challenges scientists' understanding of how planets form: low-mass stars should have less available material to form planets. Moreover, this new gas giant is on an eccentric orbit, which suggests the presence of another massive planet. The results were published in Science. Read more
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News Article NASA
January 8, 2020
This artist’s concept illustrates a catastrophic collision between two rocky exoplanets, turning both into dusty debris. Read more
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News Article EurekAlert!
February 27, 2019
Mike Brown and Konstantin Batygin offer further clues about Planet Nine. Read more
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News Article Scientific American
July 31, 2019
Why can’t astronomers decide on whether or not Pluto is a planet? Everyday Einstein explains the controversy about our faraway neighbor Read more
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News Article NASA
August 20, 2018
Our cloud-covered planet is seen from aboard the International Space Station. Read more
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News Article EurekAlert!
October 9, 2018
UZH researchers have analyzed the composition and structure of faraway exoplanets using statistical tools. Their analysis indicates whether a planet is earth-like, made up of pure rock or a water-world. The larger the planet, the more hydrogen and helium surround it. Read more
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News Article EurekAlert!
July 29, 2019
NASA's newest planet-hunting satellite has discovered a type of planet missing from our own solar system. The finding will soon allow for study of this 'missing link' between rocky Earth-like planets and gas-dominant mini-Neptunes. Read more
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