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Ultra-short period planet

Planet with an orbital period less than one day

Planet with an orbital period less than one day

An ultra-short period (USP) planet is a type of exoplanet with an orbital period 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, it is most likely that the planet has a circular orbit and is 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 for M dwarfs to for F dwarfs. USP planets seem mostly 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, however, 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 Jupiters 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 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 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. 12 known Jovian USP planets are found.

Examples

Tellurian planetsGas giant planetsNotable non-USPPs reported for reference
🜨
◌
Artist's impression
Name
*(Alternates)*Orbital period
(Days)Semimajor Axis
()KeyNotes
Kepler-974 d
*(KOI 1843.03)*0.177 (4h 15min)*Unknown*🜨
TOI-2431 b0.224 (5h 22min)0.0063 ± 0.0001🜨
Gliese 4256 b
*(TOI-6255 b)*0.2382 (5h 43min)0.0054🜨
K2-141b0.2803244 ± 0.0000015 (6h 43min)0.01064 ± 0.00016🜨
[[File:PSR J2322-2650b orbiting pulsar PSR J2322-2650.jpg112px]]PSR J2322-2650 b0.322963997 (7h 45min)0.0102◌
[[File:Lava World Kepler 78b.jpg112px]]Kepler-78b
*(KIC 8435766 b)*0.35500745(8) (8h 31min)0.00901🜨
TOI-561b0.4465697 ± 0.0000003 (10h 43min)0.01064 ± 0.00016🜨
[[File:K2-229b.jpg112px]]K2-229b0.584249 ± 0.000014 (14h 1min)0.012888 ± 0.000130🜨
[[File:TOI-2109 b.png112px]]TOI-2109b0.6725 (16h 8min)0.01791 ± 0.00065◌
[[File:Artist's impression of SPECULOOS-3 b and its host star.jpg112px]]SPECULOOS-3 b0.719 (17h 15min)0.00733🜨
[[File:Super-Earth Exoplanet 55 Cancri e (Artist’s Concept) (2024-102).png112px]][Janssen](55-cancri-e)
*(55 Cancri e)*0.7365 (17h 40min)0.01544 ± 0.00005🜨
[[File:WASP-19b.jpg112px]]Banksia
*(WASP-19b)*0.7888399 (18h 55min)0.01655 ± 0.00013◌
WASP-47e0.7896 ± 0.000011 (18h 57min)0.017🜨
--HIP 65Ab0.9809734 ± 0.0000031 (23h 32min)0.01782◌
[[File:WASP-12b (NASA).jpg112px]]WASP-12b1.0914199 ± 0.0000002 (26h 11min)0.0234

Notes

References

References

  1. "Newly-discovered planet is hot, metallic and dense as Mercury".
  2. (2018). "Kepler-78 and the Ultra-Short-Period planets". New Astronomy Reviews.
  3. (2021-08-01). "TKS X: Confirmation of TOI-1444b and a Comparative Analysis of the Ultra-short-period Planets with Hot Neptunes". The Astronomical Journal.
  4. (2022-04-01). "Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N, and TESS". Monthly Notices of the Royal Astronomical Society.
  5. "weiss".
  6. (1 March 2008). "Five Planets Orbiting 55 Cancri". [[The Astrophysical Journal]].
  7. (10 October 2010). "Radial velocity planets de-aliased. A new, short period for Super-Earth 55 Cnc e". [[The Astrophysical Journal]].
  8. (November 2022). "Revisiting the Iconic Spitzer Phase Curve of 55 Cancri e: Hotter Dayside, Cooler Nightside, and Smaller Phase Offset". [[The Astronomical Journal]].
  9. (2016). "Variability in the super-Earth 55 Cnc e". [[Monthly Notices of the Royal Astronomical Society]].
  10. (10 November 2012). "A Possible Carbon-rich Interior in Super-Earth 55 Cancri e". [[The Astrophysical Journal Letters]].
  11. (15 July 2025). "Orbital Decay of the Ultra-hot Jupiter TOI-2109b: Tidal Constraints and Transit-timing Analysis". The Astronomical Journal.
  12. (2025-07-11). "An Earth-Sized Planet in a 5.4h Orbit Around a Nearby K dwarf".
  13. (2013). "A rocky composition for an Earth-sized exoplanet". Nature.
  14. Gibney, Elizabeth. (October 30, 2013). "Exoplanet is built like Earth but much, much hotter". Nature.
  15. (April 2018). "K2-141 b A 5-M🜨 super-Earth transiting a K7 V star every 6.7 h". Astronomy & Astrophysics.
  16. (2020). "The TESS-Keck Survey II: Masses of Three Sub-Neptunes Transiting the Galactic Thick-Disk Star TOI-561". The Astronomical Journal.
  17. "Kepler-974". Caltech.
  18. (2023-09-01). "Cold Jupiters and improved masses in 38 Kepler and K2 small planet systems from 3661 HARPS-N radial velocities. No excess of cold Jupiters in small planet systems". Astronomy and Astrophysics.
  19. Martin, Pierre-Yves. (2025). "Planet TOI-6255 b".
  20. (2024). "An Earth-sized Planet on the Verge of Tidal Disruption". The Astronomical Journal.
  21. (2024-05-15). "Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3". Nature Astronomy.
  22. (2023). "A new dynamical modeling of the WASP-47 system with CHEOPS observations". Astronomy and Astrophysics.
  23. (2010). "WASP-19b: The Shortest Period Transiting Exoplanet Yet Discovered".
  24. (2013). "The secondary eclipses of WASP-19b as seen by the ASTEP 400 telescope from Antarctica". Astronomy & Astrophysics.
  25. (2017). "Detection of titanium oxide in the atmosphere of a hot Jupiter". Nature.
  26. (September 13, 2017). "Inferno World with Titanium Skies". European Southern Observatory.
  27. (2019-03-14). "New photometric analysis of five exoplanets: CoRoT-2b, HAT-P-12b, TrES-2b, WASP-12b, and WASP-52b". Monthly Notices of the Royal Astronomical Society.
  28. (2010). "WASP-12b as a prolate, inflated and disrupting planet from tidal dissipation". [[Nature (journal).
  29. [http://www.nasa.gov/mission_pages/hubble/science/planet-eater.html Hubble Finds a Star Eating a Planet] nasa.gov. 2010-05-20. Retrieved on 2010-12-10.
  30. waspplanets. (2019-11-26). "The orbit of WASP-12b is decaying".
  31. (20 January 2022). "TESS Revisits WASP-12: Updated Orbital Decay Rate and Constraints on Atmospheric Variability". The Astronomical Journal.
  32. (21 March 2018). "PSR J2322−2650 – a low-luminosity millisecond pulsar with a planetary-mass companion". Monthly Notices of the Royal Astronomical Society.
  33. (2025). "A carbon-rich atmosphere on a windy pulsar planet".
  34. "Newly-discovered planet is hot, metallic and dense as Mercury".
  35. "K2-229 b CONFIRMED PLANET OVERVIEW PAGE". NASA Exoplanet Archive.
  36. (2020). "Three short-period Jupiters from TESS". EDP Sciences.
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