4Mj Dark Star found in Triple Star System

A young ‘Dark Star’, weighing in at 4 Jupiter masses, is one of only a few such exoplanets to have been directly imaged.  It’s also a rather curious object for another reason:  It’s orbiting the main star of a triple star system some 340 light years away, in a dynamical arrangement which lies on the very edge of mathematical possibility (1).  HD131399ab is just 16 million years old, and could be classified as an ultra cool sub-brown dwarf rather than a Jovian class gas giant.  At this youthful age its temperature is about 600 degrees Celsius, allowing it to be directly imaged in infra-red by SPHERE operated by the European Southern Observatory.

The triple star system is indeed curious.  The two minor stars (B and C) orbit the main star A at a distance of about 300 Astronomical Units, all the time twirling around each other at approximately Saturn’s distance from the Sun.  The newly discovered exoplanet, HD131399ab, also orbits around the main star A in a wide orbit “about twice as large as Pluto’s if compared to our solar system, and brings the planet to about one-third of the separation of the stars [B & C] themselves.” (2).  The massive planet’s orbit around its parent star is by far the widest known orbit within a multi-star system.

This graphic shows the orbit of the planet in the HD 131399 system (red line) and the orbits of the stars (blue lines). The planet orbits the brightest star in the system, HD 131399A.

This graphic shows the orbit of the planet in the HD 131399 system (red line) and the orbits of the stars (blue lines). The planet orbits the brightest star in the system, HD 131399A.  Image Credit ESO

“The location of HD 131399Ab on a wide orbit in a triple system demonstrates that massive planets may be found on long and possibly unstable orbits in multi-star systems. HD 131399Ab is one of the lowest mass (4 ± 1 MJup) and coldest (850 ± 50 K) exoplanets to have been directly imaged.” (3)

According to the University of Arizona team’s calculations, this set-up is barely stable over the long-term, and could very easily result in the ejection of HD131399ab if any of the system’s variables were to alter.  Future data should firm up a more accurate model of this star system (2), and perhaps provide some insight into how this unusual configuration arose in the first place.  It’s certainly difficult to explain how such a massive planet, of four Jupiter masses, has found itself located twice the distance from its parent star than Pluto is from the Sun at such an early point in its lifetime.  The paper’s authors think it is likely that it initially formed closer to its parent star, and then migrated outwards.  It has managed to achieve a distance which is as close to getting flung out of the system as one could reach before HD131399ab becomes a free-floating interstellar sub-brown dwarf (1).  The authors seem torn between thinking this is a migrating system on the verge of ejection of the massive planet, or just possibly the tip of an iceberg of exotic planet-forming possibilities in outer solar system environments.  Either way, this quirky system indicates how complex wide orbit arrangements can become.

As a mental experiment, imagine a similar scenario in our own solar system where a highly distant binary Dark Star lives alongside a quite separate wide orbit Planet X object.  One might then begin to see a picture emerging of a really quite complex outer solar system configuration – one that might align with recent calculations attempting to reconcile the complex perturbing effects of the proposed Planet Nine (4, 5).


Written by

8th July 2016


1)  Ryan Mandelbaum “This could be the Strangest Exoplanet Yet” 7th July 2016 with thanks to Mart

2)  Daniel Stolte “Newly discovered planet has 3 suns” 7 July, 2016

3)  K. Wagner et al., “Direct Imaging Discovery of a Jovian Exoplanet Within a Triple Star System”, by Science, 7 July 2016

4)   Paul Seaburn “New Evidence For Existence of Planets Ten and Eleven” 15th June 2016

5)  Carlos de la Fuente Marcos, Raúl de la Fuente Marcos & Sverre J. Aarseth “Dynamical impact of the Planet Nine scenario: N-body experiments” Monthly Notices of the Royal Astronomical Society: Letters 2016, 460 (1): L123-L127 (July 21, 2016)

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