Extrasolar Planets

V

Types of Extrasolar Planets

Scientists divide the major planets found in our solar system into different categories. The inner planets Mercury, Venus, Earth, and Mars are rocky or terrestrial planets, compact worlds made of rocky materials with solid crusts and molten interiors. The outer planets Jupiter, Saturn, Uranus, and Neptune are giant worlds surrounded by thick, primitive atmospheres mainly made of hydrogen and helium. Jupiter and Saturn are called gas giants. Uranus and Neptune are sometimes called ice giants, largely made up of water in a hot, compressed state like a solid.

Planets around the other stars likely fall into some of these categories. However, many of the exoplanets astronomers have detected so far show striking differences from the Sun’s group of planets.

A

Hot Jupiters

About 40 percent of the exoplanets detected so far are so-called hot Jupiters, major gas giant planets that orbit closer to their stars than Mercury orbits the Sun. Their known masses can range from about 0.5 times up to over 8 times the mass of Jupiter (or 166 to 2,544 times the mass of Earth). Their rotation periods and their orbital periods are the same so they always keep the same face to their suns. Temperatures in their atmospheres may reach greater than 1,925°C (3,500°F). In some cases these Jupiter-like planets are more massive than Jupiter but may have similar or even smaller diameters because gravitation makes them more compact. In other cases, their atmospheres may puff out to twice the diameter of Jupiter. Studies have found water, metals, and other chemicals in the atmospheres of some hot Jupiters, as well as evidence of extremely high wind speeds.

B

Hot Neptunes

Neptune-size planets have also been found orbiting extremely close to stars. These “hot Neptunes” are about the radius of Neptune and have masses from 17 to about 22 times the mass of Earth. Such planets are thought to be mainly water in a compressed, hot solid state surrounding a rocky core, with a thin hydrogen and helium atmosphere. Neptune-size planets are probably more common than Jupiter-size planets, but are currently more difficult to detect.

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C

Super-Earths

A number of planets found around other stars must be rocky terrestrial worlds like Earth, only much more massive. These planets are sometimes called “super-Earths.” They may have ice or even liquid water on their surfaces. Some of these super-Earths may be up to 17 times the mass and 3 times the diameter of Earth—large enough to have become the cores of Jupiter-like giants. Such super-Earths probably did not accumulate hydrogen and helium gas atmospheres because they orbit cool red dwarf stars. Small stars likely formed solar systems with less gas than existed around the Sun. In other cases, the super-Earths have less than 10 times the mass of Earth—too small to have been cores of gas giant planets. One of the smallest super-Earths found so far (Gliese 581 c) has about 5 times the mass of Earth and about 1.5 times its diameter.

D

Brown Dwarfs

An object with between 13 times and 80 times the mass of Jupiter can fuse deuterium (1 proton + 1 neutron) into helium (2 protons + 2 neutrons) and release infrared radiation. These “failed stars” are called brown dwarfs. (True stars with at least 80 times the mass of Jupiter can fuse normal hydrogen [1 proton] into helium in their cores.) Brown dwarfs have been found orbiting regular stars and floating free in space. Planets and discs of dust that could form planets have been detected around brown dwarfs. A planet has been found orbiting a brown dwarf that itself orbits a regular star.