Object 116: 47 UMa

Podcast release date: 22 January 2024

Right ascension: 10:59:28.0


Epoch: ICRS

Constellation: Ursa Major

Corresponding Earth location: A location in the North Pacific Ocean over 1000 km from the island of Hokkaido, the Kamchatka Peninsula, and the Aleutian Islands

47 Ursa Majoris received its designation in the first catalog of stars created using telescope observations, which was created by the English astronomer John Flamsteed in the eighteenth century. However, Flamsteed did not actually number the stars. The stars were numbered when his catalog was translated into French [1]. In this translation, all of the stars within a constellation were given a number based on their right ascension as specified in Flamsteed's catalog, with 1 given to the star with the smallest right ascension and the highest number given to the star with the largest right ascension [1]. However, for various reasons, including the procession of the Earth's axes and the motions of nearby stars relative to background stars, these numbers today don't always correspond to the order of the stars in terms of right ascension within a constellation.

Anyhow, 47 Ursa Majoris is a Sun-like star. It is extremely Sun-like. It has the same color, it has just about the same surface temperature, it has the same spectral type, it has the very close to the same mass, and it seems like it would have close to the same luminosity [2]. It's also really close; it's located at a distance of 45.30 light years (13.89 pc) [3, 4]. Since starting this podcast, I have learned that Han Solo completed the Kessel Run in less than 12 parsecs and not less than 20 parsecs, so I'm not going to use my Star Wars sound effects for 47 Ursa Majoris.

Anyway, astronomers like to observe Sun-like stars that are really close to Earth for multiple reasons, but they observed 47 Ursa Majoris because they wanted to try to find exoplanets, or, to be more exact, they really wanted to find an Earth-like planet orbiting a Sun-like star, and indeed they found exoplanets. In fact, 47 Ursa Majoris was the third Sun-like star where a planet was found orbiting it, which was really exciting [5]. The exoplanet was discovered by R. Paul Butler and Geoff Marcy in 1996 [5]. The exoplanet, which was given the boring designation 47 Ursa Majoris b, was detected using measurements of the slight Doppler shifting of light from the star 47 Ursa Majoris caused by the gravitational force exerted by the exoplanet as it orbited the star. The exoplanet orbits its star at a distance of 2.1 Astronomical Units (AU), or 2.1 times the distance from the Sun to the Earth, which would correspond to the inner edge of the asteroid belt in our Solar System [2]. The exoplanet also has a mass 2.53 times the mass of Jupiter, which means that the exoplanet is a really large gas giant that is nothing like the Earth [2].

However, this did not stop astronomers, including R. Paul Butler and Geoff Marcy, from continuing to try to detect Earth-like exoplanets in the 47 Ursa Majoris system, and two more exoplanets were indeed found in the system. The second one, named 47 Ursa Majoris c, was discovered in 2002 [6]. This exoplanet orbits at a distance of 3.6 AU from its host star (which again would place it in our Solar System's asteroid belt) and has a mass of 0.54 times the mass of Jupiter (or, in other words, it has a mass between the masses of Jupiter and Saturn) [2]. The other, named 47 Ursa Majoris d, was discovered in 2010 [7]. It orbits at a distance of 11.6 AU from the star (which would be close to where Saturn is in our Solar System), and has a mass 1.64 times the mass of Jupiter [2]. Astute listeners will notice that none of these exoplanets are like the Earth.

However, that has not stopped astronomers from continuing to try to find Earth-like exoplanets in the 47 Ursa Majoris system and from publishing multiple theoretical works discussing how it might be really really possible that 47 Ursa Majoris specifically has an Earth-like planet orbiting at a distance of 1 AU from the star or somewhere else in the habitable zone of the system [2, 8, 9, 10, 11]. It's not immediately clear why people are so interested in finding an Earth-like exoplanet in this specific star system, especially given that lots of Earth-like planets have since been found orbiting other stars. People might be hyperfocused on 47 Ursa Majoris because it was one of the first star systems where an exoplanet was found, or maybe because people got really excited about the discovery of the second exoplanet in 2002 for some reason, or maybe because people are still really excited by studying exoplanets orbiting a Sun-like star that is really close to Earth (although, again, the distance is not as short as the Kessel Run). In any case, I would expect to see more papers published about the potential for Earth-like planets orbiting 47 Ursa Majoris in the foreseeable future, even if we end up making contact with intelligent life somewhere else, because people seem to have really high expectations for finding an Earth-like planet in the 47 Ursa Majoris system.


[1] Christensen, Lars Lindberg et al., Star Names, 2024, International Astronomical Union

[2] Cuntz, Manfred et al., Exocomets in the 47 UMa System: Theoretical Simulations Including Water Transport, 2018, Astronomical Journal, 156, 290

[3] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[4] Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533

[5] Butler, R. Paul and Marcy, Geoffrey W., A Planet Orbiting 47 Ursae Majoris, 1996, Astrophysical Journal Letters, 464, L153

[6] Fischer, Debra A. et al., A Second Planet Orbiting 47 Ursae Majoris, 2002, Astrophysical Journal, 564, 1028

[7] Gregory, Philip C. and Fischer, Debra A., A Bayesian periodogram finds evidence for three planets in 47UrsaeMajoris, 2010, Monthly Notices of the Royal Astronomical Society, 403, 731

[8] Goździewski, K., Stability of the 47 UMa planetary system, 2002, Astronomy & Astrophysics, 393, 997

[9] Laughlin, Gregory et al., A Dynamical Analysis of the 47 Ursae Majoris Planetary System, 2002, Astrophysical Journal, 579, 455

[10] Asghari, N. et al., Stability of terrestrial planets in the habitable zone of Gl 777 A, HD 72659, Gl 614, 47 Uma and HD 4208, 2004, Astronomy & Astrophysics, 426, 353

[11] Ji, Jianghui et al., Could the 47 Ursae Majoris Planetary System be a Second Solar System? Predicting the Earth-like Planets, 2005, Astrophysical Journal, 631, 1191


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)