For the past 20 years, astronomers peered into the night sky, puzzled about a type of planet called hot Jupiters. As they analyzed data from giant telescopes on mountaintops, and later data beamed to Earth from spacecraft such as the Kepler space telescope, they wondered: How did these large hot planets ever get so close to their suns?
Scientists at the University of Michigan, working with a team of colleagues, have made a startling discovery using data collected by the K2 mission: One of these mysterious hot Jupiter systems has not one, but two close-in planetary companions, leading to new clues about planet formation and migration.
Until now. Some 300 hot Jupiters have been identified over the past two decades, and this is the first time any close-in planets were discovered. This new discovery is helpful to scientists who are trying to understand how planets form and move in solar systems.
Hot Jupiters get their name because of their large size and gaseous composition (like Jupiter) and their unusually close location to their sun (hot). Large gaseous planets are believed to be formed in frigid temperatures. In our solar system that means formation must occur beyond the asteroid belt, far from the burning heat of the sun.
Hot Jupiters are invisible to the naked eye. There are some super telescopes on earth which pick them up indirectly. Scientists also study data collected from telescopes on space crafts.
Scientists made their discovery using data collected from the K-2 mission, which comes from a spacecraft holding the Kepler space telescope. The person who first spotted the possibility of a planet companion in the WASP-47 system was Hans Schwengeler, who made note of it in a public forum called Planet Hunters.
Hot Jupiters (also called roaster planets, epistellar jovians, pegasids or pegasean planets) are a class of extrasolar planets whose characteristics are similar to Jupiter, but that have high surface temperatures because they orbit very close – between approximately 0.015 and 0.5 astronomical units (2.2×106 and 74.8×106 km) – to their parent stars, whereas Jupiter orbits its parent star (the Sun) at 5.2 astronomical units (780×106 km), causing low surface temperatures.
CHARACTERSTICS OF HOT JUPITERS
They have similar characteristics to Jupiter (gas giants, usually with masses close to or exceeding that of Jupiter, which is 1.9×1027 kg); however, they orbit much more closely to the star and experience a high surface temperature.
They have a much greater chance of transiting their star as seen from a farther outlying point than planets of the same mass in larger orbits. The most notable of these are HD 209458 b, the first transiting hot Jupiter found, HD 189733 b, which was first mapped in 2007 by the Spitzer Space Telescope, and HAT-P-7b, which was recently observed by the Kepler mission.
Due to high levels of insolation they are of a lower density than they would otherwise be. This has implications for radius determination, because due to limb darkening of the planet’s background star during a transit, the planet’s ingress and egress boundaries are harder to determine.
They are all thought to have migrated to their present positions because there would not have been enough material so close to the star for a planet of that mass to have formed in situ.
Most of these have nearly circular orbits (low eccentricities). This is because their orbits have been circularized, or are being circularized, by tides.
They exhibit high-speed winds distributing the heat from the day side to the night side, thus the temperature difference between the two sides is relatively low.
They are more common around F- and G-type stars and somewhat less common around K-type stars. Hot Jupiters around red dwarfs are very rare.