K2-28

K2-28 is a metal rich M4-type main sequence star. One confirmed transiting exoplanet is known to orbit this star. There is another star 5.2 arcseconds to the north–east of K2-28 however this star has a different proper motion and is therefore physically unrelated and probably a background star.[4]

K2-28
Observation data
Epoch J2000      Equinox J2000
Constellation Aquarius[1]
Right ascension 22h 22m 29.8611s[2]
Declination −07° 57′ 19.853″[2]
Apparent magnitude (V) 16.06[3]
Characteristics
Spectral type M4V[4][5]
Apparent magnitude (J) 11.695±0.030[6]
Apparent magnitude (H) 11.028±0.023[6]
Apparent magnitude (K) 10.746±0.023[6]
Variable type Planetary transit variable[4]
Astrometry
Radial velocity (Rv)11.7[5] km/s
Proper motion (μ) RA: −254.604(41)[2] mas/yr
Dec.: −194.554(30)[2] mas/yr
Parallax (π)15.8734 ± 0.0343[2] mas
Distance205.5 ± 0.4 ly
(63.0 ± 0.1 pc)
Details[4]
Mass0.257±0.048 M☉
Radius0.288±0.028 R☉
Surface gravity (log g)4.93±0.04 cgs
Temperature3214±60 K
Metallicity [Fe/H]0.26±0.10 dex
Other designations
Gaia DR2 2622296783699476864, LP 700-6, NLTT 53655, EPIC 206318379[7]
Database references
SIMBADdata

Planetary system

Discovery

K2-28b was first noticed as a candidate extrasolar planet by Vanderburg et al. in 2016, who, in a search of 59,174 stars from the Kepler space telescope's first year of K2 observations, found 234 planetary candidates.[8] Shortly thereafter the K2-ESPRINT Project confirmed that the candidate was a super-Earth sized planet in a close orbit around a red dwarf star.[4]

K2-28 transit light curve from the Spitzer Space Telescope.[3]

Characteristics

K2-28b is a sub-Neptune sized planet orbiting its star in only 2.26 days. Despite its short orbital period the equilibrium temperature of the planet is a relatively low 500 Kelvin due to the low luminosity of the parent star.[4] Because of the very small size of the parent star this planet is a particularly favorable target for transmission spectroscopy by the James Webb Space Telescope which should be able to determine if the atmosphere is cloudy or clear by observing roughly 5 transits.[9] Among a group of small and cool planets orbiting relatively bright M-dwarfs its predicted secondary eclipse depth of 230 parts-per-million is second only to Gliese 1214 b.[3]

Secondary eclipse depth vs. temperature of small and cool planets orbiting relatively bright M-dwarfs[3]
The K2-28 planetary system[9]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
K2-28b 7.18+5.92
−3.08 (estimate) M⊕		 0.0191+0.0037
−0.0029		 2.2604455±0.0000010 0 87.1+0.90
−0.74°		 2.56+0.27
−0.26 R⊕

References
  1. Roman, Nancy G. (1987). "Identification of a Constellation From a Position". Publications of the Astronomical Society of the Pacific. 99 (617): 695–699. Bibcode:1987PASP...99..695R. doi:10.1086/132034. Vizier query form
  2. Brown, A. G. A.; et al. (Gaia collaboration). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics (in press). arXiv:2012.01533. doi:10.1051/0004-6361/202039657. S2CID 227254300. Gaia EDR3 record for this source at VizieR.
  3. Chen, Ge; et al. (2018). "An Improved Transit Measurement for a 2.4 R ⊕ Planet Orbiting A Bright Mid-M Dwarf K2–28". The Astronomical Journal. 155 (5). 223. arXiv:1801.10177. Bibcode:2018AJ....155..223C. doi:10.3847/1538-3881/aabd75.
  4. Hirano, Teruyuki; et al. (2016). "The K2-ESPRINT Project III: A Close-in Super-Earth around a Metal-rich Mid-M Dwarf". The Astrophysical Journal. 820 (1). 41. arXiv:1511.08508. Bibcode:2016ApJ...820...41H. doi:10.3847/0004-637X/820/1/41.
  5. Dressing, Courtney D.; et al. (2017). "Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. I. Classifying Low-mass Host Stars Observed during Campaigns 1–7". The Astrophysical Journal. 836 (2). 167. arXiv:1701.00586. Bibcode:2017ApJ...836..167D. doi:10.3847/1538-4357/836/2/167.
  6. Skrutskie, M. F.; et al. (2006). "The Two Micron All Sky Survey (2MASS)". The Astronomical Journal. 131 (2): 1163–1183. Bibcode:2006AJ....131.1163S. doi:10.1086/498708. Vizier catalog entry
  7. "K2-28". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-08-12.
  8. Vanderburg, Andrew; et al. (2016). "Planetary Candidates from the First Year of the K2 Mission". The Astrophysical Journal Supplement Series. 222 (1). 14. arXiv:1511.07820. Bibcode:2016ApJS..222...14V. doi:10.3847/0067-0049/222/1/14.
  9. Stefansson, Gudmundur; et al. (2018). "Diffuser-assisted Photometric Follow-up Observations of the Neptune-sized Planets K2-28b and K2-100b". The Astronomical Journal. 156 (6). 266. arXiv:1807.04420. Bibcode:2018AJ....156..266S. doi:10.3847/1538-3881/aae6ca.
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