Proximal Planet Formation

Somehow or other (and it’s by no means clear how), some exoplanet gas giants whizz around their stars at great proximity.  The hottest of these objects so far discovered is an exoplanet named Kelt-9b.  It is a sub-brown dwarf of ~3 Jupiter masses.  It’s so close to its parent star that its rotation is tidally locked, and orbits the star in just 36 hours.  The temperature of its ‘dayside’ is over 4000 degrees C.  This remarkably high temperature is likely due to the immense amount of stellar radiation Kelt-9b is subjected to.  This temperature and stellar irradiation is driving off huge amounts of hydrogen from Kelt-9b’s atmosphere, creating an extended envelope of atomic hydrogen gas (1).  Other similar tailed gas giants have been studied before (2,3).  One can only imagine how spectacular this must look – a gas giant ‘comet’ streaming out a tail from near to or even within its parent star’s extended corona.

New analysis of Kelt-9b’s atmosphere has confirmed the presence of iron and titanium atoms within the planet’s atomic chemical soup (4).  It’s known that brown dwarfs can have cloudy atmospheres containing liquid iron rain, as well as other atmospheric dusts (5).  These dusty, cloudy atmospheres tend to form below 2,500 degrees Celsius, and then clear when the brown dwarf drops its temperature below about 1,500 degrees C. 

At the kind of temperatures Kelt-9b is experiencing, its planetary gases aren’t forming clouds.  Instead, they’re dissociated into raw atomic elements.  The entire atmosphere is a gaseous stew of various atomic elements, in neutral and ionised form.  The lightest of these, hydrogen, is being constantly driven off by the bombardment of ultraviolet stellar radiation.  This atomisation of gases allows astronomers the opportunity to study the composition of ultra-hot gas giants, as their elemental composition has not become tied up in a more complex brew of compounds.  The light from the exoplanet already contains the raw elemental spectrometry.  As time goes by, the exoplanet Kelt-9b is condensing into a denser atomic stew.

I wonder whether this planet once part of the star itself?  This might seem an odd question, but the close proximity of hot Jupiters to their stars is an unexpected finding in planetary science.  They also seem to be reasonably common.  Were they planets which accreted out of the star’s protoplanetary disk, and then migrated inwards towards the stellar furnace?  Or, does the star itself somehow give birth to these proximal objects directly, as it forms?

Stars are thought to form inside egg-shaped cocoons, called dense cores, within dense molecular clouds, or stellar nurseries.  They appear to initially form as binaries within these dense cores, which then either retain their binary character, or separate into individual star systems (particularly if they started out as wide binaries) (6).  Under certain circumstances, could the gravitational pull of the binary companion cleave materials away from a star during its formation?  Or, might the binary twist the protoplanetary disk of the major into spirals, drawing out material from a star’s atmosphere to create a hot Jupiter?

We are starting to appreciate how complex protoplanetary disks can become (7).  Protoplanetary disk arms would be massive formations in their own right, and would be gravitationally powerful.  They may be pulled out by an orbiting companion object, like a brown dwarf, or massive planet located towards the tip of the arm.  At the star end of the spiral arm, the gravitational tug upon the star itself may contribute towards slowly spooling out a hot Jupiter object.  Material can also stream towards a star within a binary system, moving between outer and inner disks (8,9).  This process may also contribute towards building up an in situ ultra-hot planet very close to a star, prior to the exclusion of the binary companion.

Perhaps in the case of the sub-brown dwarf Kelt-9b we are witnessing a snapshot of a late stage in the process of proximal planet formation, taken some time after the ejection of a prior binary companion.


Written by Andy Lloyd

16th August 2018


1)   Fei Yan & Thomas Henning “An extended hydrogen envelope of the extremely hot giant exoplanet KELT-9b”. Nature Astronomy, 2 July 2018,

2)  A. Lecavelier des Etangs et al. “Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b”. Astron. & Astrophysics, December 2012, 543: L4

3)  B. Lavie et al. “The long egress of GJ 436b’s giant exosphere”. Astron. & Astrophysics, September 2017, 605: L7

4)  H. J. Hoeijmakers et al. “Atomic iron and titanium in the atmosphere of the exoplanet KELT-9b” 15 August 2018,

5)  I. Crossfield et al. “A Global Cloud Map of the Nearest Known Brown Dwarf” 2014

6)  Sarah Sadavoy & Steven Stahler “Embedded binaries and their dense cores” Monthly Notices of the Royal Astronomical Society, 21 August 2017, 469(4): pp3881–3900,

7)  Joshua Sokol “Stellar Disks Reveal How Planets Get Made” 21 May 2018

8)  Hamish Johnston “Planet-forming stream found in binary star system” 31 Oct 2014,

9)  Anne Dutrey et al. “Possible planet formation in the young, low-mass, multiple stellar system GG Tau A” Nature, 30 October 2014, 514: pp 600–602

Image 1 Credit: NASA/JPL-Caltech

Image 2 Credit: NASA/ESA/ESO/M. Benisty et al.

Image 3 Credit: ESO/L Calçada



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