Kepler-37, also known as UGA-1785,[7][8][9] is a G-type main-sequence star located in the constellationLyra209 light-years (64 parsecs) from Earth. It is host to exoplanetsKepler-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-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 (2,402mi) 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 700K (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 5.4ME,[14] but a 2023 study instead found an upper limit on its mass of only 2ME.[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 (238,900mi) 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
↑ Masses more than a few times that of the Moon result in unphysically high densities.
1 2 Rajpaul, V. M.; Buchhave, L. A.; Lacedelli, G.; Rice, K.; Mortier, A.; Malavolta, L.; Aigrain, S.; Borsato, L.; Mayo, A. W.; Charbonneau, D.; Damasso, M.; Dumusque, X.; Ghedina, A.; Latham, D. W.; López-Morales, M.; Magazzù, A.; Micela, G.; Molinari, E.; Pepe, F.; Piotto, G.; Poretti, E.; Rowther, S.; Sozzetti, A.; Udry, S.; Watson, C. A. (2021), "A HARPS-N mass for the elusive Kepler-37d: A case study in disentangling stellar activity and planetary signals", Monthly Notices of the Royal Astronomical Society, 507 (2): 1847–1868, arXiv:2107.13900, Bibcode:2021MNRAS.507.1847R, doi:10.1093/mnras/stab2192
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