An ultra-short period (USP) planet is a type of

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

with an

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of less than one Earth day. At this short distance, tidal interactions lead to relatively rapid orbital and spin evolution. Therefore when there is a USP planet around a mature

main-sequence star In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...

it is most likely that the planet has a

circular orbit A circular orbit is an orbit with a fixed distance around the barycenter; that is, in the shape of a circle. In this case, not only the distance, but also the speed, angular speed, Potential energy, potential and kinetic energy are constant. T ...

and is

tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...

. There are not many USP planets with sizes exceeding 2 Earth radii. About one out of 200 Sun-like stars ( G dwarfs) has an ultra-short-period planet. There is a strong dependence of the occurrence rate on the mass of the host star. The occurrence rate falls from (1.1 ± 0.4)% for M dwarfs to (0.15 ± 0.05)% for F dwarfs. Mostly the USP planets seem consistent with an Earth-like composition of 70% rock and 30% iron, but K2-229b has a higher density suggesting a more massive iron core. WASP-47e and 55 Cnc e have a lower density and are compatible with pure rock, or a rocky-iron body surrounded by a layer of water (or other volatiles). A difference between

hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter (i.e. Jupiter analogue, Jupiter analogues) but that have very short orbital periods (). The close proximity to t ...

s and terrestrial USP planets is the proximity of planetary companions. Hot Jupiters are rarely found with other planets within a factor of 2–3 in orbital period or distance. In contrast, terrestrial USP planets almost always have longer-period planetary companions. The period ratio between adjacent planets tends to be larger if one of them is a USP planet suggesting the USP planet has undergone tidal orbital decay which may still be ongoing. USP planets also tend to have higher mutual

inclinations Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

with adjacent planets than for pairs of planets in wider orbits, suggesting that USP planets have experienced inclination excitation in addition to orbital decay. There are several known giant planets with a period shorter than one day. Their occurrence must be lower by at least an order of magnitude than that of terrestrial USP planets. It had been proposed that USP planets were the rocky cores of evaporated hot Jupiters, however the

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

of the host stars of USP planets is lower than that of hot Jupiters' stars so it seems more likely that USP planets are the cores of evaporated gas dwarfs. A study by the TESS-Keck Survey using 17 USP planets found that USP planets predominantly have an Earth-like compositions with iron core mass of about 32% and have masses below runaway accretion. USP are also almost always found in multiple-planet systems around stars with solar metallicity.

Examples

TOI-561b

Studies of TOI-561b found that it is an USP planet with the lowest density (3.8 ± 0.5 g cm⁻³) as of April 2022. The low density of this planet is explained with a massive

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

layer, no H/He envelope, as well as a predicted water

steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

atmosphere. The steam atmosphere could be detected with JWST in the future. More complex models might be needed to fully explain the unusual properties of TOI-561b.

Kepler-78b

K2-141b

References

{{exoplanet Types of planet