Ultra-short period planet

Artist's impression of a Jovian ultra-short period planet orbiting a star

Artist impression of tellurian ultra-short period planet

An ultra-short period (USP) planet is a type of exoplanet with an orbital period of less than one Earth day. ^[1] 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. ^[1] 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. ^[1] 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). ^[1]

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. ^[1]

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. ^[1]

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. ^[1]

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. ^[2] 12 known Jovian USP planets are found. ^[3]

Examples

Key (Classification)

🜨	Tellurian planets
◌	Gas giant planets

Key (Illustration)

	Artist's impression

Illustration	Name (Alternates)	Orbital period (Days)	Semimajor Axis ( AU )	Key	Notes
	Kepler-974 d (KOI 1843.03)	0.177 (4h 15min) [4]	–	🜨	Shortest orbit around a main-sequence star (an M dwarf). [4]
	TOI-2431 b	0.224 (5h 22 min) [5]	0.0063 ± 0.0001 [5]	🜨	Sixth-shortest for any known exoplanet, as of 2025. In about 31 million years, it will enter the roche lobe of the host star and be torn appart. [5]
	Gliese 4256 b (TOI-6255 b)	0.2382 (5.72 hours) [6]	0.0054 [6]	🜨	Gliese 4256 b orbits so close to the host star that it has nearly passed the roche limit, causing the planet to be pulled into an egg-like shape. It is predicted that in around 400 million years, the planet will be ripped apart and engulfed by the star. [7]
	K2-141b	0.2803244 ± 0.0000015 [8] (6.7 hours)	0.01064 ± 0.00016 [8]	🜨	Shortest-period planet with a precisely determined mass. [9]
	Kepler-78b (KIC 8435766 b)	0.35500745(8) [8] (8.52 hours)	0.00901 +0.00012 −0.00019 [8]	🜨	Kepler-78b orbits extremely close that the planet's surface is estimated to be at a temperature of 2,200  K (1,930  °C ; 3,500  °F ) [8] which is high enough to have stripped the planet of any stable atmosphere, but the liquid and solid portions of the planet should be stable [10] that the planet is Earth-sized lava planet. [11]
	TOI-2109b	0.6725 (16.14 hours) [3]	0.01791 ± 0.00065 [3]	◌	Shortest orbital period among the hot Jupiters, at 0.6725 days (16.14 hours), and the highest rotational rate as well as the largest size and mass among the 12 known Jovian ultra-short period planets. [3]
	TOI-561b	0.4465697 ± 0.0000003 [12] (11 hours)	0.01064 ± 0.00016 [12]	🜨	Lowest density of USP planet (3.8 ± 0.5 g cm−3) as of April 2022. The low density of this planet is explained with a massive water layer, no hydrogen nor helium envelope, as well as a predicted water steam atmosphere. The steam atmosphere could be detected with the James Webb Space Telescope in the future. More complex models might be needed to fully explain the unusual properties of TOI-561b. [13]
	SPECULOOS-3 b	0.719(17h 15min) [14]	0.00733 [14]	🜨
	Janssen (55 Cancri e)	0.7365 (17.68 hours) [15]	0.01544 ± 0.00005 [15]	🜨	When Janssen was discovered, it was first thought to take about 2.8 days to orbit 55 Cancri (Copernicus). [16] It was later corrected to 0.7365 d (17.68 h), making it the shortest orbital period at that time. [15] Due to its close proximity to the star, 55 Cancri e is extremely hot, with temperatures on the day side exceeding 3,000 Kelvin. [17] The planet's thermal emission is observed to be variable, possibly as a result of volcanic activity. [18] It has been proposed that 55 Cancri e could be a carbon planet. [19]
	WASP-47e	0.789592 ± 0.000011 [20]	0.017 [20]	🜨	WASP-47e orbits closer than the hot Jupiter WASP-47 b, which is not expected for planetary systems containing Hot Jupiters, as a migrating gas giant is thought to kick out any small inner planets. In order for the system to come out the way it is now, the two gas giants (WASP-47 b and WASP-47c) likely would have to have formed before the lower-mass planets WASP-47e and WASP-47d. This is called two-stage planetary formation, and it is hypothesized that WASP-47b would have moved inwards and brought planet-forming material close to the star. Once most of the gas dissipates, the two gas-poor planets form nearby the large Hot Jupiter. [21]

References

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2. Dai, Fei; Howard, Andrew W.; Batalha, Natalie M.; Beard, Corey; Behmard, Aida; Blunt, Sarah; Brinkman, Casey L.; Chontos, Ashley; Crossfield, Ian J. M.; Dalba, Paul A.; Dressing, Courtney; Fulton, Benjamin; Giacalone, Steven; Hill, Michelle L.; Huber, Daniel (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. 162 (2): 62. arXiv: 2105.08844 . Bibcode:2021AJ....162...62D. doi: 10.3847/1538-3881/ac02bd . ISSN   0004-6256. S2CID   234778143.
3. Jaime, A. Alvarado-Montes; Mario, Sucerquia; Jorge, I. Zuluaga; Christian, Schwab (15 July 2025). "Orbital Decay of the Ultra-hot Jupiter TOI-2109b: Tidal Constraints and Transit-timing Analysis". The Astronomical Journal. 988 (1): 66. arXiv: 2505.18941 . Bibcode:2025ApJ...988...66A. doi: 10.3847/1538-4357/ade057 .
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8. Bonomo, A. S.; Dumusque, X.; Massa, A.; Mortier, A.; Bongiolatti, R.; Malavolta, L.; Sozzetti, A.; Buchhave, L. A.; Damasso, M.; Haywood, R. D.; Morbidelli, A.; Latham, D. W.; Molinari, E.; Pepe, F.; Poretti, E. (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. 677: A33. arXiv: 2304.05773 . Bibcode:2023A&A...677A..33B. doi:10.1051/0004-6361/202346211. ISSN   0004-6361.
9. Barragán, O.; Gandolfi, D.; Dai, F. (April 2018). "K2-141 b A 5-M_🜨 super-Earth transiting a K7 V star every 6.7 h". Astronomy & Astrophysics. 612. A95. arXiv: 1711.02097 . doi: 10.1051/0004-6361/201732217 . S2CID   119473627.
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11. Gibney, Elizabeth (October 30, 2013). "Exoplanet is built like Earth but much, much hotter". Nature. doi:10.1038/nature.2013.14058 . Retrieved March 4, 2022.
12. Weiss, Lauren M.; et al. (2020). "The TESS-Keck Survey II: Masses of Three Sub-Neptunes Transiting the Galactic Thick-Disk Star TOI-561". The Astronomical Journal. 161 (2) 56. arXiv: 2009.03071 . Bibcode: 2021AJ....161...56W . doi: 10.3847/1538-3881/abd409 . S2CID   229279232.
13. Lacedelli, G.; Wilson, T. G.; Malavolta, L.; Hooton, M. J.; Collier Cameron, A.; Alibert, Y.; Mortier, A.; Bonfanti, A.; Haywood, R. D.; Hoyer, S.; Piotto, G.; Bekkelien, A.; Vanderburg, A. M.; Benz, W.; Dumusque, X. (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. 511 (3): 4551–4571. arXiv: 2201.07727 . Bibcode:2022MNRAS.511.4551L. doi: 10.1093/mnras/stac199 . ISSN   0035-8711.
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16. Fischer, Debra A.; Marcy, Geoffrey W.; Butler, R. Paul; Vogt, Steven S.; Laughlin, Greg; Henry, Gregory W.; Abouav, David; Peek, Kathryn M. G.; Wright, Jason T.; Johnson, John A.; McCarthy, Chris; Isaacson, Howard (1 March 2008) [23 December 2007 (v1)]. "Five Planets Orbiting 55 Cancri". The Astrophysical Journal . 675 (1): 790–801. arXiv: 0712.3917 . Bibcode:2008ApJ...675..790F. doi:10.1086/525512. S2CID   55779685.
17. Mercier, Samson J.; Dang, Lisa; et al. (November 2022). "Revisiting the Iconic Spitzer Phase Curve of 55 Cancri e: Hotter Dayside, Cooler Nightside, and Smaller Phase Offset". The Astronomical Journal . 164 (5): 204. arXiv: 2209.02090 . Bibcode:2022AJ....164..204M. doi: 10.3847/1538-3881/ac8f22 .
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