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Notional representations of a “hot Jupiter” orbiting very close to its host star (above, left) and an icy “super Earth” a few times larger than the Earth (above, right). These types of planets have been found around other stars but are not present in the Solar System.
Astronomers have found many planets around other stars both near and far. Some of these “exoplanets” resemble planets in the Solar System, but others are very different. Up to 160 billion planets might exist just in our own galaxy, the Milky Way. We have even found a small hot exoplanet orbiting Alpha Centauri B, the star system nearest to the Sun (four light years away).
HABITABLE ZONES: The Solar System’s habitable zone stretches from near Venus to slightly beyond Mars (top). The habitable zone of a smaller, cooler star like Gliese 581 is closer to the star (middle)
The habitable zone is the “Goldilocks region” around a star, where the surface of an Earth-like planet receives the amount of light needed to bring its temperature into the range where water is liquid (0 to 100 °C).
Diagrams of two exoplanet systems found by NASA’s Kepler mission are compared to the Solar System (middle). All three systems have planets in their habitable zones, which are marked by the patterned region. However, these Kepler planets are much more massive than Earth.
PLANETARY TRANSITS: The red curve shows a dip in a star’s brightness as a planet passes in front during transit. The amount of light blocked reveals the size of the planet. Other studies reveal its mass, density and temperature, for comparison with planets in our Solar System.
The transit method is a highly productive strategy for discovering and characterizing exoplanets. When an exoplanet “transits,” or passes in front of its star, it blocks a fraction of the star’s light and causes a dip in brightness. Since larger planets block more light, the size of the dip gives the size of the planet. Transits can also be used to probe planetary atmospheres by searching for evidence of gases such as water and methane. Scientists at Goddard are using transits to determine the size, structure and composition of these distant worlds.
Goddard scientists and engineers develop telescopes for exoplanet science. Top: NASA’s James Webb Space Telescope, now being built. Bottom: Notional telescope and starshade for imaging Earth-sized planets in the habitable zones of nearby stars.
Most exoplanets are too distant to see in pictures taken with present telescopes, but scientists study their sizes, compositions and atmospheres using other methods.