Object 7: 30 Arietis

Podcast release date: 04 November 2019

Right ascension: 02:36:59.1


Epoch: ICRS

Constellation: Aires

Corresponding Earth location: The road between Buwat and Al Mindassah to the northwest of Medina, Saudi Arabia

30 Arietis (also called 30 Ari) is a rather complex thing located in the constellation Aires. Although this type of designation is used for individual stars, it's much more than just a single star.

With even a small telescope, it is possible to see that 30 Ari can be separated into two components. The one to the east is called 30 Ari A, and the one to the west is 30 Ari B. 30 Ari A has an apparent magnitude of about 6.5, and 30 Ari B has an apparent magnitude of about 7.1 [1], so while these two objects would not be visible to the naked eye, they would be visible in a small telescope. As seen from Earth, 30 Ari A and 30 Ari B are separated by 38 arcseconds [2], which is about the size of Saturn's rings as seen from Earth. 30 Ari A and 30 Ari B are at a distance of about 147 light years (45 parcsec) [3,4], which means that the two objects are separated by a distance of about 1720 AU. Other astronomers have determined that 30 Ari A and 30 Ari B appear physically close enough to each other that they are probably gravitationally bound and orbiting a common center of mass [2], although they would be expected to take tens of thousands of years to orbit each other [1]. However, the most recent distance measurements to 30 Ari A and 30 Ari B differ very slightly but just enough to be statistically significant. This difference in distances would separate 30 Ari A and 30 Ari B by more than a light year, although it is possible that the uncertainties in the new measurements are slightly higher than what has been reported and that the two objects are actually close enough to be orbiting each other.

If you noticed, I have resisted referring to 30 Ari A and 30 Ari B as stars. 30 Ari A is actually a binary star system that is so close together that the only way that it could be identified as a binary system was by using spectroscopy to measure the Doppler shifting of the stars, which demonstrated that the stars were moving slightly in a way that would be consistent with the two stars being in orbit around each other. The stars have an orbital period of about 26.6 hours or 1.11 days [5]. The larger star is an F-type main sequence star [1], which means that it is a star that fuses hydrogen into helium in its core like the Sun but that it is slightly hotter, larger, and brighter than the Sun. The smaller star has not been identified, probably because it is very faint in comparison to the brighter star, but it is probably some type of red dwarf, a star that is much smaller than the Sun but that still fuses hydrogen into helium in its core.

30 Ari B is also an F-type main sequence star [1], and up until 10 years ago, everybody thought that it was just a single star. The star was then targeted by an exoplanet survey that observed the Doppler shifting of the star for over five-and-a-half years before accumulating enough data to show that an exoplanet was orbiting the star [1]. The exoplanet, named 30 Ari Bb, has an average orbital radius of close to 1 AU and period of about 345 Earth days [6], which is why it took so long identify that the planet was there. Its mass is at least 6.6 times the mass of Jupiter [6], which means that it is a gas giant, although this does not preclude the possibility that it may have various moons. However, the temperatures on any such moons could be over about 50 degrees Celsius (or about 90 degrees Fahrenheit) warmer than the Earth, so it could be a little difficult for Earth-like life to survive on the surface of any such exomoons.

The exoplanet researchers were not satisfied with their Doppler shifting measurements, so they also got some new images of 30 Ari B with the Keck Observatory and found the presence of another star in the 30 Ari B system. The new star lies about half an arcsecond away from 30 Ari B [6]. This is close enough that blurring by the atmosphere would blend the light of the two stars together, but the Keck Observatory uses a system called adaptive optics that can remove this blurring. This new star is a red dwarf and orbits 30 Ari B outside the orbit of the exoplanet at a distance of about 24 AU, which in our solar system would place the star somewhere between the orbits of Uranus and Neptune. The orbital period is uncertain, but the lower limit is around 100 years [6].

So, to recap, 30 Ari consists of five objects divided into two objects called 30 Ari A and 30 Ari B. 30 Ari A consists of two stars, a yellow-white star and a red dwarf, in a close binary orbit where the two stars go around each other every 1.11 days. 30 Ari B consists of a yellow-white star with a massive gas giant planet orbiting it once every 345 days and a red dwarf orbiting it once every 100 years or more. The 30 Ari A and 30 Ari B systems may be gravitationally bound to each other, in which case they would orbit around a common centre of mass once every several tens of thousands of years.

All of these results demonstrate that exoplanets can be found in rather unusual stellar systems. While astronomers might have once been biased towards finding exoplanets orbiting single stars like the Sun, 30 Ari shows that it is possible for exoplanets to be found in very exotic and unusual multiple stellar systems, and these results have encouraged astronomers to search for such exoplanets in other exotic stellar systems and to make modifications to their models of how exoplanets form.


[1] Guenther, E. W. et al., A substellar component orbiting the F-star 30 Arietis B, 2009, Astronomy & Astrophysics, 507, 1659

[2] Shatsky, N., The long-period companions of multiple stars tend to have moderate eccentricities, 2001, Astronomy & Astrophysics, 380, 238

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

[4] Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy & Astrophysics, 616, A1

[5] Morbey, C. L. and Brosterhus, E. B., A Search for Spectroscopic Binaries from Published Radial Velocity Data, 1974, Publications of the Astronomical Society of the Pacific, 86, 455

[6] Kane, Stephen R. et al., On the Stellar Companion to the Exoplanet Hosting Star 30 Arietis B, 2015, Astrophysical Journal, 815, 32


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: benjaminharveydesign, Dalibor, ivolipa, jameswrowles, milton., Mr_Maistro, shoba, UncleSigmund, and Xulie at The Freesound Project

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