Artist's impression of the nonexistentFomalhaut b, an exoplanet directly observed by theHubble telescopePlanetFomalhaut b (inset againstFomalhaut's interplanetary dust cloud) imaged by theHubble Space Telescope's coronagraph (NASA photo)Discovery image of theGliese 758 system, taken withSubaru telescope in the near infrared. It is unclear whether the companion should be regarded as a planet or abrown dwarf.2MASS J044144 is abrown dwarf with a companion about 5-10 times the mass of Jupiter. It is not clear whether this companion object is a sub-brown dwarf or a planet.Exoplanet discoveries by year
In 2013, estimates of the number ofterrestrial planets in theMilky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by theKepler space observatory.[3] Among them are 461Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbedKepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbitbrown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
Analogies with planets in the Solar System apply to few of the extrasolar planets known. Most are quite unlike any of our planets, for example the so-called "hot Jupiters".
In the sixteenth century, the Italian philosopherGiordano Bruno, an early supporter of theCopernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the HolyInquisition.[8]
In the eighteenth century, the same possibility was mentioned byIsaac Newton in hisPrincipia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion ofOne".[9]
The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 2002.
In 1992, radio astronomers announced the discovery of planets around apulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of thesupernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores ofgas giants that survived the supernova and then decayed into their current orbits.
On October 6, 1995, Michel Mayor and Didier Queloz of theUniversity of Geneva announced the first definitive detection of an exoplanet orbiting an ordinarymain-sequence star (51 Pegasi).[12] This discovery, made at theObservatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolutionspectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring theirgravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watchingoccultations when a star becomes dimmer as an orbiting planetpassed in front of it.
In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
They may support life. One recently discovered exoplanet,Gliese 581g is thought to possibly support life, but the existence of this planet is not yet confirmed.
Classes of exoplanets include:
Asuper-terran orsuper-Earth is a terrestrial planet that is bigger than Earth, but smaller thanNeptune
Awater-world is a hypothetical class of planet between terrestrial and jovian. These worlds would have very very little, if any dryland.
Hot Jupiters are gas planets orbiting closer to their stars thanMercury to the sun.
The nearest star with planets isAlpha Centauri. It is 4.3light years away. Using standardrockets, it would take tens of thousands of years to get there.[14] The nearest star similar to our Sun isTau Ceti. It has five planets, one of which in thehabitable zone, where liquid water may exist.[15][16]
Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) tozero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.
↑"Cosmos" inThe New Encyclopædia Britannica (15th edition, Chicago, 1991)16:787:2a. "For his advocacy of an infinity of suns and earths, he was burned at the stake in 1600."
↑Campbell B.; Walker G.A.H.; Yang S. 1988. A search for substellar companions to solar-type stars.Astrophysical Journal331: 902–921. Bibcode 1988ApJ...331..902C. doi:10.1086/166608
↑Wolszczan A. & Frail D.A. 1992. A planetary system around the millisecond pulsar PSR1257+12.Nature355 (6356): 145–147. doi:10.1038/355145a0
