HD 21749

HD 21749

HD 21749 – star in the constellation Reticulum
Observation data Epoch J2000      Equinox J2000
Constellation	Reticulum
Right ascension	03^h 26^m 59.22^s[1]
Declination	−63° 29′ 56.9″[1]
Apparent magnitude (V)	8.143[2]
Characteristics
Spectral type	K4.5V[3]
U−B color index	1.144[2]
B−V color index	1.159[2]
Astrometry
Radial velocity (Rv)	59.32±0.12[1] km/s
Proper motion (μ)	RA: 355.20[1] mas/yr Dec.: -247.39[1] mas/yr
Parallax (π)	61.2271 ± 0.0150 mas[1]
Distance	53.27 ± 0.01 ly (16.333 ± 0.004 pc)
Absolute magnitude (MV)	6.95[4]
Details[5]
Mass	0.73±0.07 M☉
Radius	0.695±0.030 R☉
Luminosity	0.20597±0.00016 L☉
Surface gravity (log g)	4.613+0.052 −0.061 cgs
Temperature	4640±100 K
Metallicity [Fe/H]	0.003±0.060 dex
Rotation	34.1+2.4 −2.7 d[6]
Rotational velocity (v sin i)	1.04 km/s
Age	3.8±3.7 Gyr
Other designations
CD−63 110, CPD−63 232A, GJ 143, HIP 16069, SAO 248808, WDS J03270-6330A, TYC 8870-01392-1, 2MASS J03265922-6329569, TOI-186[7]
Database references
SIMBAD	data

HD 21749 is an orange main-sequence star in the constellation Reticulum. It has an apparent visual magnitude of 8.143, which means it is too dim to be seen with the naked eye. From parallax measurements by the Gaia spacecraft, it is located 53 ly (16 pc) from Earth.

In 2019, it was discovered that the star has two exoplanets: a possibly rocky, hot sub-Neptune-sized exoplanet named HD 21749 b; and an Earth-sized exoplanet named HD 21749 c. These exoplanets were discovered by the TESS spacecraft.

Stellar characteristics

HD 21749 is a K-type main sequence star (orange dwarf) with a spectral type of K4.5V,^[3] indicating it is smaller and cooler than the Sun. It is estimated to have a mass of 0.73 M_☉, a radius of 0.70 R_☉, and a luminosity of 0.20 L_☉. Its effective temperature is 4,640 K, which gives the star an orange color typical of K-type stars. Its metallicity—the proportion of elements other than hydrogen and helium—is approximately equal to the Sun's.^[5]

This star is moderately active, as shown by its spectral activity indicators and photometric data. These measurements indicate a rotation period of around 30 to 40 days, with a most likely value of 34 days. Stellar activity also creates radial velocity variations, which complicates the measurement of the mass of the planets in the system.^[6]

A companion to HD 21749 is listed in double star catalogues, a 9th-magnitude star separated by 22″ in 2015, although decreasing rapidly due to the high proper motion of the primary.^[8] The companion is much more distant than the primary and the two are unrelated except being coincidentally in the same line of sight.^[9]

Planetary system

Size comparison of the two known planets of HD 21749 (artistic concept) with Earth

In January 2019, the discovery of an exoplanet around HD 21749 was published. The planet was identified from a single transit event detected by the TESS spacecraft, using data from the first two observation sectors of the mission. Since a single transit is insufficient to determine the orbit of a planet, astronomers used archival radial velocity data from the HARPS spectrograph to detect the planet's signal, which allowed the determination of its orbital period and mass.^[10] In April 2019, with two additional months of data from the TESS spacecraft, the orbital period of the planet was confirmed with the observation of new transits, and a second planet was discovered.^[5]

HD 21749 c

The inner planet, HD 21749 c (the second in order of discovery), is orbiting the star at a distance of 0.08 AU with a period of just 7.8 days. A terrestrial planet, it has a radius of 1.1 R_🜨 and was the first Earth-sized planet found by TESS.^[6][5] Its mass is too low to be calculated with current radial velocity data, with an upper limit of 3.5 M_🜨; a probabilistic model estimates it is most likely between 1 and 2 M_🜨.^[6]

HD 21749 b

The outer planet, HD 21749 b, orbits the star at a distance of 0.21 AU with a period of 35.6 days. With a mass of 20 M_🜨 and a radius of 2.9 R_🜨, it is similar to Neptune but much denser. Its density of 4.7 g/cm³ suggests it is composed of a substantial rocky core, with a radius of approximately 2.1 R_🜨, plus a relatively thick gaseous layer.^[6]

The HD 21749 planetary system^[6]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
c	<3.5 M🜨	0.076±0.008	7.7902+0.0004 −0.0006	0	89.44+0.36 −0.52°	1.13+0.11 −0.10 R🜨
b	20.0±2.7 M🜨	0.209+0.022 −0.021	35.6133+0.0005 −0.0006	0.164+0.062 −0.058	89.40+0.07 −0.08°	2.86+0.21 −0.20 R🜨

References

1. 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.
2. Koen, C.; et al. (21 April 2010), "UBV(RI)_CJHK observations of Hipparcos-selected nearby stars", Monthly Notices of the Royal Astronomical Society, 403 (4): 1949–1968, Bibcode:2010MNRAS.403.1949K, doi:10.1111/j.1365-2966.2009.16182.x
3. Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637, S2CID 119476992
4. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644
5. Dragomir, Diana; et al. (2019), "TESS Delivers Its First Earth-sized Planet and a Warm Sub-Neptune", The Astrophysical Journal, 875 (2): L7, arXiv:1901.00051, Bibcode:2019ApJ...875L...7D, doi:10.3847/2041-8213/ab12ed, S2CID 129945503
6. Gan, Tianjun; Wang, Sharon Xuesong; Teske, Johanna K.; Mao, Shude; Howard, Ward S.; Law, Nicholas M.; Batalha, Natasha E.; Vanderburg, Andrew; Dragomir, Diana; Huang, Chelsea X.; Feng, Fabo; Butler, R. Paul; Crane, Jeffrey D.; Shectman, Stephen A.; Beletsky, Yuri; Shporer, Avi; Montet, Benjamin T.; Burt, Jennifer A.; Feinstein, Adina D.; Flowers, Erin; Nandakumar, Sangeetha; Barbieri, Mauro; Corbett, Hank; Ratzloff, Jeffrey K.; Galliher, Nathan; Chavez, Ramses Gonzalez; Vasquez, Alan; Glazier, Amy; Haislip, Joshua (2021), "Revisiting the HD 21749 planetary system with stellar activity modelling", Monthly Notices of the Royal Astronomical Society, 501 (4): 6042, arXiv:2012.04873, Bibcode:2021MNRAS.501.6042G, doi:10.1093/mnras/staa3886
7. "HD 21749". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 8 January 2018.
8. Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001), "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog", The Astronomical Journal, 122 (6): 3466, Bibcode:2001AJ....122.3466M, doi:10.1086/323920
9. 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.
10. Trifonov, Trifon; Rybizki, Jan; Kürster, Martin (2019), "TESS exoplanet candidates validated with HARPS archival data. A massive Neptune around GJ 143 and two Neptunes around HD 23472", Astronomy and Astrophysics, 622: 622, arXiv:1812.04501, Bibcode:2019A&A...622L...7T, doi:10.1051/0004-6361/201834817, S2CID 119347462

External links

• Staff (2019). "Stellar Overview Page - HD 21749". NASA Exoplanet Archive. Retrieved 8 January 2019.
• Overbye, Dennis (7 January 2019). "Another Day, Another Exoplanet: NASA's TESS Keeps Counting More". The New York Times. Retrieved 8 January 2019.
• Massachusetts Institute of Technology (16 April 2019). "TESS discovers its first Earth-sized planet - Orbiting a nearby star, the new planet is the smallest identified so far by the TESS mission". EurekAlert!. Retrieved 16 April 2019.