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HAT-P-26 / Guahayona
Observation data; Epoch J2000 Equinox J2000
Constellation	Virgo
Right ascension	14h 12m 37.53311s
Declination	+04° 03′ 36.1166″
Apparent magnitude (V)	11.76
Characteristics
Evolutionary stage	Main sequence
Spectral type	K1V
Astrometry
Radial velocity (Rv)	14.10±0.39 km/s
Proper motion (μ)	RA: 37.735 mas/yr ; Dec.: -142.816 mas/yr
Parallax (π)	6.9985 ± 0.0204 mas
Distance	466 ± 1 ly ; (142.9 ± 0.4 pc)
Details
Mass	0.816±0.033 M☉
Radius	0.788+0.098; −0.043 R☉
Luminosity	0.38+0.16; −0.06 L☉
Surface gravity (log g)	4.56±0.06 cgs
Temperature	5079±88 K
Metallicity	−0.04±0.08
Rotational velocity (v sin i)	1.8±0.5 km/s
Age	9.0+3.0; −4.9 Gyr
Other designations
	Guahayona, Gaia DR2 3668036348641580288, TYC 320-1027-1, GSC 0320-01027, 2MASS J14123753+0403359
Database references
SIMBAD	data

HAT-P-26 is a K-type main-sequence star located about 466 light-years (143 parsecs) away in the constellation of Virgo. A survey in 2015 did not find any stellar companions in orbit around it,^[4] although a red dwarf companion with a temperature 4000+100
−350 K is suspected on wide orbit.^[5]

Nomenclature

The designation HAT-P-26 indicates that this was the 26th star found to have a planet by the HATNet Project.

In August 2022, this planetary system was included among 20 systems to be named by the third NameExoWorlds project.^[6] The approved names, proposed by a team from Puerto Rico, were announced in June 2023. HAT-P-26 is named Guahayona and its planet is named Guataubá, after figures from Taíno mythology.^[7]

Planetary system

In 2010 a transiting hot Neptune planet was detected.^[3] The transiting planet HAT-P-26b was detected by the HATNet Project using telescopes located in Hawaii and Arizona. The planet is likely formed by pebble accretion mechanism.^[8] The transmission spectrum of HAT-P-26b was taken in 2015, with the best fit favouring either a cloudless atmosphere or an atmosphere with a low-lying cloud deck.^[9] The atmospheric composition of the planet was measured in 2019, and a water vapor volume fraction of 1.5^+2.1
_−0.9% was detected. HAT-P-26 is carbon depleted, with a C/O ratio constrained to less than 0.33. Also, the planet's atmosphere contains light metal hydrides.^[10] The measured planetary temperature is equal to 563⁺⁵⁸
₋₅₄ K.

In 2023, the atmosphere of the planet was found to contain 2.4%+2.9%
−1.6% water vapor at a temperature of 590+60
−50 K.^[11]

A 2019 study detected transit-timing variations (TTVs) of HAT-P-26b, the cause of which was unclear at that time.^[12] With more data by 2023, it was suggested that the TTVs may be caused by a second planet in the system.^[11] In 2024, a candidate second planet, about twice the size of Earth and slightly farther from the star than planet b, was detected by transit in TESS data, but requires further observations to be fully confirmed.^[13]

The HAT-P-26 planetary system^[3]^[13]
Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b / Guataubá	0.059±0.007 $M$ _J	0.0479±0.0006	4.234516(15)	0.124±0.060	88.6+0.5 −0.9°	0.565+0.072 −0.032 $R$ _J
c (unconfirmed)	4.8+3.5 −2.0^[a] M_🜨	—	6.594+0.009 −0.007	<0.09	—	1.97±0.20 R_🜨

Notes

^ This mass is predicted based on mass-radius relationships. If this planet is responsible for the observed TTVs it should be possible to measure its mass that way; the 2023 TTV study found a mass of about 6.36 Earths based on an assumed period of 8.47 days,^[11] but if the period is actually shorter at 6.59 days the mass would be different.

References

^ ^a ^b ^c ^d ^e Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
^ ^a ^b HAT-P-26 -- High proper-motion Star
^ ^a ^b ^c ^d Hartman, J. D.; Bakos, G. Á.; Kipping, D. M.; Torres, G.; Kovács, G.; Noyes, R. W.; Latham, D. W.; Howard, A. W.; Fischer, D. A.; Johnson, J. A.; Marcy, G. W.; Isaacson, H.; Quinn, S. N.; Buchhave, L. A.; Béky, B.; Sasselov, D. D.; Stefanik, R. P.; Esquerdo, G. A.; Everett, M.; Perumpilly, G.; Lázár, J.; Papp, I.; Sári, P. (2010), "HAT-P-26b: A LOW-DENSITY NEPTUNE-MASS PLANET TRANSITING A K STAR", The Astrophysical Journal, 728 (2): 138, arXiv:1010.1008, Bibcode:2011ApJ...728..138H, doi:10.1088/0004-637X/728/2/138, S2CID 119228956
^ Wöllert, Maria; Brandner, Wolfgang; Bergfors, Carolina; Henning, Thomas (2015), "A Lucky Imaging search for stellar companions to transiting planet host stars", Astronomy & Astrophysics, 575: A23, arXiv:1507.01938, Bibcode:2015A&A...575A..23W, doi:10.1051/0004-6361/201424091, S2CID 119250579
^ Piskorz, Danielle; Knutson, Heather A.; Ngo, Henry; Muirhead, Philip S.; Batygin, Konstantin; Crepp, Justin R.; Hinkley, Sasha; Morton, Timothy D. (2015), "Friends of Hot Jupiters. III. An Infrared Spectroscopic Search for Low-Mass Stellar Companions", The Astrophysical Journal, 814 (2): 148, arXiv:1510.08062, Bibcode:2015ApJ...814..148P, doi:10.1088/0004-637X/814/2/148, S2CID 11525988
^ "List of ExoWorlds 2022". nameexoworlds.iau.org. IAU. 8 August 2022. Retrieved 27 August 2022.
^ "2022 Approved Names". nameexoworlds.iau.org. IAU. Retrieved 7 June 2023.
^ Mohamad Ali-Dib, Gunjan Lakhlani, "Possible formation pathways for the low-density Neptune-mass planet HAT-P-26b"
^ Stevenson, Kevin B.; Bean, Jacob L.; Seifahrt, Andreas; Gilbert, Gregory J.; Line, Michael R.; Désert, Jean-Michel; Fortney, Jonathan J. (2015), "A SEARCH FOR WATER IN THE ATMOSPHERE OF HAT-P-26b USING LDSS-3C", The Astrophysical Journal, 817 (2): 141, arXiv:1511.08226, doi:10.3847/0004-637X/817/2/141, S2CID 118394486
^ MacDonald, Ryan J.; Madhusudhan, Nikku (2019), "The Metal-Rich Atmosphere of the Neptune HAT-P-26b", Monthly Notices of the Royal Astronomical Society, 486: 1292–1315, arXiv:1903.09151, doi:10.1093/mnras/stz789, S2CID 85459516
^ ^a ^b ^c A-thano, Napaporn; Awiphan, Supachai; Jiang, Ing-Guey; Kerins, Eamonn; Priyadarshi, Akshay; McDonald, Iain; Joshi, Yogesh C.; Chulikorn, Thansuda; Hayes, Joshua J. C.; Charles, Stephen; Huang, Chung-Kai; Rattanamala, Ronnakrit; Yeh, Li-Chin; Vik S Dhillon (2023), "Revisiting the Transit Timing and Atmosphere Characterization of the Neptune-mass Planet HAT-P-26 b", The Astronomical Journal, 166 (6): 223, arXiv:2303.03610, Bibcode:2023AJ....166..223A, doi:10.3847/1538-3881/acfeea
^ von Essen, C.; Wedemeyer, S.; Sosa, M. S.; Hjorth, M.; Parkash, V.; Freudenthal, J.; Mallonn, M.; Miculán, R. G.; Zibecchi, L.; Cellone, S.; Torres, A. F. (2019), "Indications for transit timing variations in the exo-Neptune HAT-P-26b", Astronomy & Astrophysics, 628: A116, arXiv:1904.06360, Bibcode:2019A&A...628A.116V, doi:10.1051/0004-6361/201731966, S2CID 118674293
^ ^a ^b Dévora-Pajares, Martín; Pozuelos, Francisco J.; et al. (July 2024). "The SHERLOCK pipeline: new exoplanet candidates in the WASP-16, HAT-P-27, HAT-P-26, and TOI-2411 systems". Monthly Notices of the Royal Astronomical Society. arXiv:2407.14602. doi:10.1093/mnras/stae1740.