Kepler-37

Kepler-37, also known as UGA-1785,^[7][8][9] is a G-type main-sequence star located in the constellation Lyra 209 light-years (64 parsecs) from Earth. It is host to exoplanets Kepler-37b, Kepler-37c, Kepler-37d and possibly Kepler-37e, all of which orbit very close to it. Kepler-37 has a mass about 80.3 percent of the Sun's and a radius about 77 percent as large.^[5] It has a temperature similar to that of the Sun, but a bit cooler at 5,357 K. It has about half the metallicity of the Sun. With an age of roughly 6 billion years,^[10] it is slightly older than the Sun, but is still a main-sequence star. Until January 2015, Kepler-37 was the smallest star to be measured via asteroseismology.^[11]

Kepler-37

Line up comparing the planets in the Kepler-37 system to the Moon and planets in the Solar System.
Observation data Epoch J2000      Equinox J2000
Constellation	Lyra[1]
Right ascension	18h 56m 14.30760s[2]
Declination	+44° 31′ 05.3896″[2]
Apparent magnitude (V)	9.710[3]
Characteristics
Spectral type	G8V
Astrometry
Radial velocity (Rv)	−30.92±0.20[2] km/s
Proper motion (μ)	RA: −60.396 mas/yr[2] Dec.: 48.657 mas/yr[2]
Parallax (π)	15.6253±0.0105 mas[2]
Distance	208.7 ± 0.1 ly (64.00 ± 0.04 pc)
Details
Mass	0.79+0.033 −0.03[4] M☉
Radius	0.789+0.0064 −0.0056[4] R☉
Temperature	5357±68[4] K
Metallicity [Fe/H]	−0.36±0.05[4] dex
Rotational velocity (v sin i)	1.1 (± 1.1)[5] km/s
Age	7.6+3.4 −3.1[4] Gyr
Other designations
KOI-245, KIC 8478994,[3] TYC 3131-1199-1, BD+44 3020, 2MASS J18561431+4431052, GSC 03131-01199, Gaia DR2 2106674071344722688[6]
Database references
SIMBAD	data
KIC	data

Planetary system

The Kepler-37 planetary system^[4]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	<0.79[a] M🜨	0.1019±0.0014	13.367020(60)	<0.098	88.63+0.30 −0.53°	0.3098+0.0059 −0.0076 R🜨
c	<1.3 M🜨	0.1390±0.0020	21.301848(18)	<0.099	89.07+0.19 −0.33°	0.755+0.033 −0.055 R🜨
d	<2.0 M🜨	0.2109±0.0030	39.7922622(65)	<0.10	89.335+0.043 −0.047°	2.030+0.030 −0.039 R🜨
e[12] (disputed)	≥8.1±1.7 M🜨	0.25	50.25±0.15	—	—	—

Kepler-37b is the closest planet to Kepler-37. At the time of its discovery in February 2013, it was the smallest known exoplanet.^[13] At 3,865 kilometres (2,402 mi) in diameter, it is slightly larger than the Moon.^[13] It orbits Kepler-37 once every 13 days at a distance of about 0.1 astronomical units (AU).^[5] Kepler-37b has a rocky surface and is believed to be too small and too close to its star to support water or maintain an atmosphere.^[13] Surface temperature is estimated at 700 K (427 °C; 800 °F).^[11]

Kepler-37c is around three-quarters of the diameter of Earth and orbits approximately every 21 days at a distance of just under 0.14 AU. Kepler-37d is about twice the diameter of Earth. It orbits in around 40 days at a distance of nearly 0.21 AU.^[5] Neither are able to support liquid water due to their proximity to Kepler-37.^[13]

A 2021 study detected Kepler-37d via radial velocity, finding a mass of about 5.4 M_🜨,^[14] but a 2023 study instead found an upper limit on its mass of only 2 M_🜨.^[4] In either case, it is not a rocky planet, but a low-density planet rich in volatiles. The periods of the three inner planets are close (within one per cent) to a 5:8:15 mean-motion resonance relationship.

In 2015, a grant was approved to further expand the Sagan Planet Walk by installing a Kepler-37d station on the Moon 384,500 kilometers (238,900 mi) away.^[15]

Discovery

The Kepler-37 planets were discovered in September 2012 with the aid of transit events detected by the Kepler space telescope, and announced to the public in February 2013.^[5] Computer simulation was used to rule out other astronomical phenomena mimicking planetary transits with probabilities of error <0.05% (3σ) for each potential planet. Additionally, simulation demonstrated that the proposed planetary configuration was stable.^[5] The exoplanets were considerably smaller than any previously detected, leading Science World Reports to state that "a major technological improvement for the telescope" had been achieved.^[13]

Thomas Barclay, an astrophysicist on the Kepler space telescope team, said the discovery was "really good news" in the search for hospitable planets, a prime objective of the project, because it demonstrated the telescope was capable of detecting Earth-sized planets.^[16] However, he does not anticipate finding many planets as small as Kepler-37b due to the very small amount of light such planets obscure.^[16] According to NASA scientist Jack Lissauer, the discovery of Kepler-37b "suggests such little planets are common, and more planetary wonders await as we continue to gather and analyze additional data."^[11] Astronomer John Johnson of Caltech university said the discovery would have been "unimaginable" a few years ago and that the telescope had revolutionized astronomers' picture of the universe.^[16]

The asteroseismology work was, in part, paid for by the Nonprofit Adopt a Star program operated by White Dwarf Research Corporation, a crowd funded non-profit organization.^[17]

In 2014, a fourth planet with an orbital period of 51 days (Kepler-37e) was reported based on transit-timing variations.^[18] Previously this signal was thought to be a false positive due to its low signal-to-noise ratio, and indeed later studies failed to detect either the transit or TTV signal. A study in 2021 again found that the TTV data disfavors the presence of planet e, and argued that it should be stripped of its "confirmed planet" status.^{[14]: 3–4, 18–19}

A 2023 study modeled the system both with and without a planet candidate at 51 days. Based on the assumption that a planet with a circular orbit of about 51 days is present, marginal radial velocity evidence was found for a sub-Neptune mass planet. Evidence of a longer-period planet candidate was also found. No additional planet has been confirmed, and the system remains with three confirmed planets.^{[12]: 37–38}

Notes

1. Masses more than a few times that of the Moon result in unphysically high densities.