Astronomy:42 Draconis

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Short description: Star in the constellation Draco
42 Draconis / Fafnir
Location of 42 Draconis (circled in red)
Observation data
Equinox J2000.0]] (ICRS)
Constellation Draco[1]
Right ascension  18h 25m 59.13696s[2]
Declination +65° 33′ 48.5313″[2]
Apparent magnitude (V) 4.82[3]
Characteristics
Spectral type K1.5 III[3]
B−V color index 1.18[1]
Astrometry
Radial velocity (Rv)31.75±0.12[2] km/s
Proper motion (μ) RA: +105.816[2] mas/yr
Dec.: −26.846[2] mas/yr
Parallax (π)11.056 ± 0.0841[2] mas
Distance295 ± 2 ly
(90.4 ± 0.7 pc)
Absolute magnitude (MV)−0.10[1]
Details[4]
Mass1.07±0.01 M☉
Radius19.78±0.17 R☉
Luminosity129.26+1.77
−1.05 L☉
Surface gravity (log g)1.9±0.02 cgs
Temperature4,449+11
−17 K
Metallicity [Fe/H]−0.43±0.01 dex
Rotation554±142 d
(~479 or ~690 d)
Rotational velocity (v sin i)1.76±0.45 km/s
Age9.49±1.76[5] Gyr
13.19±1.92[3] Gyr
Other designations
Fafnir, BD+65°1271, GC 25212, HD 170693, HIP 90344, HR 6945, SAO 17888, PPM 20916
Database references
SIMBADdata
Exoplanet Archivedata
Extrasolar Planets
Encyclopaedia		data

42 Draconis (abbreviated 42 Dra), formally named Fafnir (/ˈfɑːvnər/ or /ˈfɑːfnɪər/),[6][7] is a 5th magnitude K-type giant star located approximately 295 light-years away in the constellation of Draco. An exoplanet (designated 42 Draconis b, later named Orbitar) was once thought to be orbiting the star, but its existence has since been refuted.[4]

Of spectral type K1.5III, the star has a mass similar to the Sun but with a radius 22 times greater. It is a metal-poor star with metallicity as low as 35% that of the Sun and its age is 9.49 billion years. It is the southern pole star of Venus.[8]

A 2019 study using Gaia DR2 data found a companion star to 42 Draconis, about half the Sun's mass, at a separation of 24 arcseconds, corresponding to 2220 AU.[9]

Nomenclature

42 Draconis in optical light

42 Draconis is the star's Flamsteed designation. Following its discovery the planet was designated 42 Draconis b. In July 2014 the International Astronomical Union launched NameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[10] The process involved public nomination and voting for the new names.[11] In December 2015, the IAU announced the winning names were Fafnir for this star and Orbitar for its planet.[12]

The winning names were submitted by the Brevard Astronomical Society of Brevard County, Florida, United States.[13] Fafnir was a Norse mythological dwarf who turned into a Germanic dragon. It is also the name of a fictional planet in Larry Niven's Known Space universe of similar description, ('Draco' is Latin for 'dragon'); Orbitar is a contrived word paying homage to the space launch and orbital operations of NASA.[14]

In 2016, the IAU organized a Working Group on Star Names (WGSN)[15] to catalog and standardize proper names for stars. In its first bulletin of July 2016,[16] the WGSN explicitly recognized the names of exoplanets and their host stars approved by the Executive Committee Working Group Public Naming of Planets and Planetary Satellites, including the names of stars adopted during the 2015 NameExoWorlds campaign. This star is now so entered in the IAU Catalog of Star Names.[7]

Disproven planet

"Orbitar" redirects here. For the video game weapon, see Kid Icarus: Uprising.

The discovery of 42 Draconis b (later named Orbitar) by the radial velocity method was announced in 2009. It was thought to be a super-Jupiter, with a minimum mass 4 times that of Jupiter, an orbital period of 479 days, and an eccentricity of 0.38.[5] However, the existence of this planet was questioned in a 2021 study by the same authors, which found that more recent radial velocity measurements were inconsistent with the proposed planetary orbit.[17] With further observations, by 2025 the planet hypothesis could be conclusively ruled out.[4] The star appears to have intrinsic radial velocity variations that mimic a planetary companion, as observed in other giant stars such as Gamma Draconis and Aldebaran.[4]

In a 2024 study, 42 Draconis b was listed as one of several doubtful planets around giant stars, along with other named planets around 14 Andromedae and 41 Lyncis.[18] With the 2025 refutation, it is now the second named exoplanet to be conclusively disproven, after Fomalhaut b (Dagon) in 2020.[19]

See also

References

  1. ↑ 1.0 1.1 1.2 Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters 38 (5): 331. doi:10.1134/S1063773712050015. Bibcode: 2012AstL...38..331A  XHIP record for this object at VizieR.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  3. ↑ 3.0 3.1 3.2 Ligi, R. et al. (February 2016), "Radii, masses, and ages of 18 bright stars using interferometry and new estimations of exoplanetary parameters", Astronomy & Astrophysics 586: 23, doi:10.1051/0004-6361/201527054, A94, Bibcode: 2016A&A...586A..94L. 
  4. ↑ 4.0 4.1 4.2 4.3 Hatzes, Artie P. et al. (May 2025). "No Planet around the K Giant Star 42 Draconis". Astronomy & Astrophysics 699: A260. doi:10.1051/0004-6361/202554197. Bibcode: 2025A&A...699A.260H. 
  5. ↑ 5.0 5.1 Döllinger, M. P. et al. (2009). "Planetary companion candidates around the K giant stars 42 Draconis and HD 139 357". Astronomy and Astrophysics 499 (3): 935–942. doi:10.1051/0004-6361/200810837. Bibcode: 2009A&A...499..935D. http://www.aanda.org/articles/aa/full_html/2009/21/aa10837-08/aa10837-08.html. 
  6. ↑ "Fafnir". Merriam-Webster Dictionary. https://www.merriam-webster.com/dictionary/Fafnir. 
  7. ↑ 7.0 7.1 "IAU Catalog of Star Names". http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt. 
  8. ↑ ""Is Polaris the north star for all the other planets just like it is for Earth?" | Planetarium | University of Southern Maine". https://usm.maine.edu/planet/polaris-north-star-all-other-planets-just-it-earth. 
  9. ↑ Mugrauer, M. (December 2019). "Search for stellar companions of exoplanet host stars by exploring the second ESA-Gaia data release". Monthly Notices of the Royal Astronomical Society 490 (4): 5088–5102. doi:10.1093/mnras/stz2673. Bibcode: 2019MNRAS.490.5088M. 
  10. ↑ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  11. ↑ "NameExoWorlds The Process". http://nameexoworlds.iau.org/process. 
  12. ↑ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  13. ↑ Website
  14. ↑ "NameExoWorlds The Approved Names". http://nameexoworlds.iau.org/names. 
  15. ↑ "IAU Working Group on Star Names (WGSN)". https://www.iau.org/science/scientific_bodies/working_groups/280/. 
  16. ↑ "Bulletin of the IAU Working Group on Star Names, No. 1". http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf. 
  17. ↑ Döllinger, M. P.; Hartmann, M. (September 2021). "A Sanity Check for Planets around Evolved Stars". The Astrophysical Journal Supplement Series 256 (1): 10. doi:10.3847/1538-4365/ac081a. Bibcode: 2021ApJS..256...10D. 
  18. ↑ Spaeth, Dane et al. (September 2024). "Non-radial oscillations mimicking a brown dwarf orbiting the cluster giant NGC 4349 No. 127". Astronomy & Astrophysics 689: A91. doi:10.1051/0004-6361/202450163. Bibcode: 2024A&A...689A..91S. 
  19. ↑ Gáspár, András; Rieke, George H. (April 20, 2020). "New HST data and modeling reveal a massive planetesimal collision around Fomalhaut". PNAS 117 (18): 9712–9722. doi:10.1073/pnas.1912506117. PMID 32312810. Bibcode: 2020PNAS..117.9712G. 


Coordinates: Sky map 18h 25m 59.1381s, +65° 33′ 48.530″



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