The host star has a temperature of 10,170 K, which is remarkable, as transiting planets are not usually detected in orbit of hot stars; as a reference, only six A-type stars were known to have planets prior to KELT-9b discovery (WASP-33, an A-type star with a temperature of 7,430 K being the hottest at the moment of the discovery of KELT-9b), and no B-type stars were known (hotter than A-type stars); KELT-9, classified as B9.5-A0[ or A1,[9] is the first B-type star (or, at least, almost B-type) in which a planet has been discovered.]
KELT-9b was detected using the Kilodegree Extremely Little Telescope and the results of its unusual nature were published in 2017.[10]
The planet was found to have a largely extended hydrogen atmosphere.[12] The size of the atmosphere is close to the planetary Roche lobe, which implies the planetary atmosphere is escaping at a high mass loss rate. The discovery was made by probing the Balmer Hα line absorption when the planet is transiting in front of its host star.[12]
It is tidally locked.[1]
The atmosphere of the planet is so hot that refractory elements that would usually be sequestered into molecules can exist as atomic species, including neutral and singly ionized atomic iron (Fe and Fe+) and singly ionized titanium (Ti+).[15]
KELT-9b has surprised astronomers yet again with the discovery that its atmosphere contains the metals iron and titanium.
It is known as a Hot Jupiter – which gives a hint to its nature. Nearly two times the size of Jupiter. It is even hotter than the surface of some stars. In some ways it straddles the line between a star and a gas-giant exoplanet.
References
1. ^[https://www.jpl.nasa.gov/news/news.php?feature=6865 NASA JPL, Pasadena CA (5 June 2017) Astronomers Find Planet Hotter Than Most Stars]
2. ^1 {{cite journal|bibcode=1981A&AS...45..455J|title=Spectral Classification in the MK System of 167 Northern HD Stars|journal=Astronomy and Astrophysics Supplement|volume=45|pages=455|author1=Jensen|first1=K. S.|year=1981|url=http://adsabs.harvard.edu/full/1981A%26AS...45..455J|access-date=2018-08-18}}
3. ^1 {{cite journal|bibcode=2016DDA....4720403C|title=KELT-9b: A Case Study in Dynamical Planet Ingestion by a Hot Host Star|journal=American Astronomical Society|volume=47|pages=204.03|author1=Collins|first1=Karen A.|last2=Stassun|first2=Keivan|last3=Gaudi|first3=B. Scott|last4=Beatty|first4=Thomas G.|last5=Zhou|first5=George|last6=Latham|first6=David W.|last7=Bieryla|first7=Allyson|last8=Eastman|first8=Jason D.|last9=Siverd|first9=Robert|last10=Crepp|first10=Justin R.|last11=Pepper|first11=Joshua|year=2016|url=http://adsabs.harvard.edu/abs/2016DDA....4720403C|access-date=2018-08-18}}
4. ^1 2 {{Cite journal|doi=10.1038/s41550-018-0503-3|title=An extended hydrogen envelope of the extremely hot giant exoplanet KELT-9b|journal=Nature Astronomy|last1=Yan|first1=Fei|last2=Henning|first2=Thomas|arxiv = 1807.00869 |date=2 July 2018|url=https://www.researchgate.net/publication/326129899_An_extended_hydrogen_envelope_of_the_extremely_hot_giant_exoplanet_KELT-9b |accessdate=18 August 2018 |language=En |issn=2397-3366|bibcode=2018NatAs.tmp...86Y}}
5. ^1 {{cite journal |last1=Hoeijmakers |first1=H. Jens |last2=Ehrenreich |first2=David |last3=Heng |first3=Kevin |last4=Kitzmann |first4=Daniel |last5=Grimm |first5=Simon L. |last6=Allart |first6=Romain |last7=Deitrick |first7=Russell |last8=Wyttenbach |first8=Aurélien |last9=Oreshenko |first9=Maria |last10=Pino |first10=Lorenzo |last11=Rimmer |first11=Paul B. |last12=Molinari |first12=Emilio |last13=Di Fabrizio |first13=Luca |title=Atomic iron and titanium in the atmosphere of the exoplanet KELT-9b |journal=Nature |date=15 August 2018 |pmid=30111838|doi=10.1038/s41586-018-0401-y|bibcode=2018Natur.560..453H|arxiv=1808.05653|url=https://www.nature.com/articles/s41586-018-0401-y|accessdate=18 August 2018|issn=1476-4687|pages=1}}