Object 126: HD 181433

Podcast release date: 10 June 2024

Right ascension: 19:25:09.6


Epoch: ICRS

Constellation: Pavo

Corresponding Earth location: Around 37 km from Adelaide Island near the Antarctic Peninsula

The coordinates for this episode point to a location in the southern constellation Pavo, which is supposed to represent a peacock. This was one of 12 constellations invented in the sixteenth century by two Dutch navigators named Pieter Dirkszoon Keyser and Frederick de Houtman to fill up what had previously been an uncharted part of the southern sky, at least from the European perspective [1]. A peacock seems kind of random, but, apparently, Keyser and de Houtman liked naming random patches of stars in the sky after random animals they encountered in the tropics.

The specific object that this episode's coordinates point to is the star system HD 181433, which is located at a distance of 88.1 light years (27.0 pc) [2, 3]. The single star at the center of this system is a ho-hum red dwarf with an age of about 6.7 billion years [4, 5], making it about a couple billion years older than the Sun. However, the one notable thing about this star system is that three exoplanets have been detected orbiting it. These planets were discovered in observations performed in 2005 and published in 2009 [6]. Two common techniques are used to detect exoplanets. One technique involves watching for the faint dimming that occurs when planets pass in front of the star, but this technique only works if the orbit of the planet is aligned in just the right way for this to happen. The other technique involves looking for the slight, periodic Doppler shifting of light from a star that would indicate that planets are orbiting it, and while this is more time consuming to do, it's generally more effective at detecting planets because the orbits of the planets do not need to have any specific alignment. It's this second technique that was used to find the exoplanets orbiting HD 181433.

The three exoplanets are labelled HD 181433 b, c, and d. Exoplanet b is what is called a super-Earth, which is a class of exoplanets in between the size of the Earth and a small gas giant like Uranus or Neptune. It's about 7 times the mass of the earth, orbits its star once every 9.4 days at an average distance of 0.08 AU (where 1 AU is the distance from the Earth to the Sun) [7]. A super-Earth might sound very exciting, but astronomers have discovered a lot of super-Earths at this point, and HD 181433 b is so close to its host star that it's too hot to harbor life, and it's actually not that interesting, so I'm not going to mention this exoplanet again during the rest of this episode.

Exoplanets c and d are both gas giants; c is 0.67 times the mass of Jupiter, and d is 0.61 times the pass of Jupiter, or, in other words, both have masses in between the masses of Saturn and Jupiter [7]. Exoplanet c orbits at an average distance of 1.819 AU, which would be kind-of outside the orbit of Mars in our Solar System, with an orbital period of 2.78 years [7]. Exoplanet d orbits at an average distance of 6.60 AU, which would be slightly wider than Jupiter's orbit, with an orbital period of 19.2 years [7]. So far, all of these measurements are not that unusual. However, the orbits of exoplanets c and d are rather elongated, which is very unusual for any type of planet-sized objects. (Actually, the orbit of exoplanet b is also elongated, but like I said before, I'm not going to mention it again during the rest of this podcast.)

The elongation of any planet's orbit can be described by a parameter called eccentricity that varies between 0 and 1. A value of 0 is equivalent to a circular orbit, while a value close to 1 is a very, very elongated orbit that almost looks like a straight line. The orbits of most of the planets in our Solar System have eccentricities less than 0.1 [8], which means that they are very circular, although Mercury's is 0.21 [8], which means that it's a little more oval-shaped, and Pluto's is 0.24 [8], which also makes it more oval-shaped, but we knew that Pluto's orbit was weird anyway, and besides, Pluto isn't classified as a real planet anymore [9]. The orbits of some of the most famous comets in the Solar System's have eccentricites that typically range from 0.5 to 0.999 [10]. So, for comparison, the orbit of HD 181433 c has an eccentricity of 0.24, which is very similar to Pluto's orbit, and the orbit of HD 181433 d has an eccentricity of 0.47, which is similar to that of a comet [7]. That is a bit weird. In fact, the ellipticity of d is so weird that it has been very challenging to properly calculate the exoplanet's orbit. Some earlier calculations of d's orbit had suggested that that exoplanet actually passes within 0.1 AU of c's orbit [11], which could result in gravitational instabilities in the system, and that would have been unexpected for a star system billions of years old. However, those calculations were simply inaccurate; c and d actually stay further than 1 AU away from each other [7]. Also, it's worth mentioning that, as of the time of this recording, d has barely completed a single orbit around its host star, so further observations and analysis of its orbit are needed to confirm that it's orbit does indeed have a very high eccentricity.

Overall, the HD 181433 system has been an excellent example of how some exoplanets might have really elongated and unusual orbits, and it has forces astronomers to improve their techniques for measuring exoplanets' orbits. The lessons learned from HD 181433 should lead to fewer false alarms about unstable planetary systems. Additionally, I think the really elliptical orbits of these exoplanets may have some sort of implications for what was happening in this star system when its exoplanets were forming. It seems to me like it would be a bit difficult to get a really elongated exoplanet orbit like d's orbit without some sort of gravitational interaction or collision or something during ther planet formation process, and I would be interested to see some sort of scientific analysis of this published in the future.


[1] Ridpath, Ian, Star tales, 1988

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

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

[4] van Leeuwen, F., Validation of the new Hipparcos reduction, 2007, Astronomy & Astrophysics, 474, 653

[5] Trevisan, M. et al., Analysis of old very metal rich stars in the solar neighbourhood, 2011, Astronomy & Astrophysics, 535, A42

[6] Bouchy, F. et al., The HARPS search for southern extra-solar planets. XVII. Super-Earth and Neptune-mass planets in multiple planet systems HD 47 186 and HD 181 433, 2009, Astronomy & Astrophysics, 496, 527

[7] Horner, Jonathan et al., The HD 181433 Planetary System: Dynamics and a New Orbital Solution, 2019, Astronomical Journal, 158, 100

[8] William, David R., Planetary Fact Sheet, 2024, NASA Space Science Data Coordinated Archive

[9] Lindberg Christensen, Lars, Pluto and the Developing Landscape of Our Solar System, 2024, International Astronomical Union

[10] William, David R., Comet Fact Sheet, 2016, NASA Space Science Data Coordinated Archive

[11] Campanella, Giammarco, Treating dynamical stability as an observable: a 5:2 mean motion resonance configuration for the extrasolar system HD 181433, 2011, Monthly Notices of the Royal Astronomical Society, 418, 1028


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: almalaut, chaosportal, ivolipa, jameswrowles, morganpurkis, thecityrings, thenudo, and Xulie at The Freesound Project

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