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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

Resource Type
Trusted Partner Content
News Article Scientific American
August 15, 2018
About 80 percent of the Earth's biomass is plant life, with humans about equal to krill way down the heft chart.     Read more
Trusted Partner Content
News Article EurekAlert!
December 13, 2018
In their quest to learn more about faraway planets, astronomers discovered that a medium-sized planet roughly the size of Neptune is evaporating at a rate 100 times faster than a previously discovered planet of similar size. Read more
Trusted Partner Content
News Article EurekAlert!
July 25, 2019
A newly-discovered planet orbiting one of the brightest young stars known could provide valuable information on how planetary bodies form. Read more
Trusted Partner Content
News Article EurekAlert!
February 20, 2020
A signal originally detected by the Kepler spacecraft has been validated as an exoplanet using the Habitable-zone Planet Finder. Read more
Trusted Partner Content
News Article Scientific American
September 6, 2019
Spin-offs from space science and exploration offer eco-friendly benefits for Earth Read more
Trusted Partner Content
News Article Scientific American
April 17, 2019
Showers of interstellar debris could speed up the assembly of worlds around young stars Read more
Trusted Partner Content
News Article EurekAlert!
July 29, 2019
NASA's Transiting Exoplanet Survey Satellite, or TESS, has discovered three new worlds that are among the smallest, nearest exoplanets known to date. The planets orbit a star just 73 light years away and include a small, rocky super-Earth and two sub-Neptunes -- planets about half the size of our own icy giant. Read more
Trusted Partner Content
News Article Scientific American
August 22, 2019
Thirty years after a probe visited Neptune, many scientists say now is the time to finally return to that world and Uranus Read more
Trusted Partner Content
News Article EurekAlert!
May 28, 2019
An exoplanet smaller than Neptune with its own atmosphere has been discovered in the Neptunian Desert, by an international collaboration of astronomers, with the University of Warwick taking a leading role. Read more
Trusted Partner Content
News Article EurekAlert!
February 27, 2020
University of British Columbia astronomy student Michelle Kunimoto has discovered 17 new planets, including a potentially habitable, Earth-sized world, by combing through data gathered by NASA's Kepler mission. Read more
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