We’re used to thinking about small asteroid-like bodies emitting gaseous tails: They’re called comets. But could there be such things as cometary planets? Two studies regarding the escaping atmospheres of hot exoplanets have been published this month in the journal Science (1,2). The first is a transiting warm Neptune-mass exoplanet located 20 times nearer its host star than the Earth is to the Sun. The tail of helium being blasted away from this planet by radiation from its orange dwarf host star extends some five planetary radii out. The planet, known as HAT-P-11b, is blown up like a helium balloon, according to the researchers who have been studying it (3).
However, HAT-P-11 is not a young star still blasting away at the primordial atmosphere of a new Neptune-sized world, as you might expect. Instead, HAT-P-11 is 6.5 billion years old; almost 2 billion years old than our own Sun. So, why is it still managing to have such a devastating effect upon the Neptune-sized exoplanet in its midst? Common sense would dictate that you can’t have such an effect going on for 6.5 billion years, as the planet would have been eradicated long ago. Loosely bound helium held in this gaseous ‘envelope’ would surely leak out into space in considerable quantities over time? Like with comets repeatedly transiting around their stars at perihelion, you would think that at some point the volatile gases would all get blown away. Perhaps HAT-P-11b was once a much greater hot Jupiter world which has shrunk to Neptune proportions over time. Or, perhaps this is a case of inwards migration of this world from further out in the star system.
A similar tail of atmospheric helium is being blasted back from another giant exoplanet, this time known as WASP-69b (5). This world is about a quarter of the mass of Jupiter, making it a sub-‘hot Jupiter’ object. The discovery of this helium tail was also made using the Carmenes instrument, installed on the 3.5-meter telescope of the Calar Alto Observatory in Spain. In this case, the host star, is 2 billion years old and just a little smaller than our own Sun (6), making it another orange dwarf. So, again, this effect isn’t due to the young nature of the system – there is a sustained effect taking place over a long period of time if the system has been static for that entire time. How long can such helium leakage be sustained from this ‘evaporating exoplanet’? Previous discoveries of such leaking exoplanets have included a ‘hot Neptune’ planet whizzing around a ~9 billion year old red dwarf, Gliese 436 (7).
Evidently, these hydrogen and helium ‘envelopes’ can be remarkably long-lasting. But that is not always the case. Other ‘hot’ Neptunes may shed so much atmospheric material that they end up as smaller planets. This is thought to be the case for the hot Neptune planet GJ 3470b, whose superheated atmosphere is evaporating off at a rate 100 times faster than that of Gliese 436b (8). During its 2 billion lifetime, it may already have lost a third of its mass to the radiation pressure of its star, and within several billion more years, it could be left a mere rocky, terrestrial world (one would assume that as the planet loses mass, the process of atmospheric evaporation would accelerate – there is less gravity to keep it held to the bombarded world). The knowledge that GJ 3470b is losing its atmosphere so quickly suggests that other hot Neptunes may have eroded down to smaller, rocky super-Earths. This effect might then explain a the relative dearth of such objects among exoplanets (9). Smaller exoplanets which have been discovered close to their parent red dwarf stars also seem to have been shorn of their atmospheres by powerful flares and coronal mass ejections – a phenomenon which red dwarfs seem quite prone towards (14).
Why is this important? Because it indicates that planets can appear like comets, under the right circumstances. The volatile gases wrapped up in the atmospheres of Neptune-sized worlds can become drawn out into what must be rather fabulous looking envelopes of gas. These effects are being noticed in the specific cases of exoplanets which are located very close to their stars. This is reasonable in the sense that the planets are subjected to the ultraviolet radiation and stellar winds at this proximity. But, the reason we’re seeing this in these cases is that the planets are transiting the stars, allowing measurement of their extended atmospheres to be taken. that creates bias towards very close planets.
It may also be true that Neptune-sized planets located further out from these orange and red dwarfs could be exuding atmospheric hydrogen and helium. Observational bias dictates that we only see the effect with their closer exoplanetary cousins. This potentially means that Neptune-sized worlds on eccentric, comet-like trajectories might be highly spectacular in nature during their perihelion passages – true cometary planets.
An Extraordinary Binary Coupling
More massive objects can form very close to stars, too. Sometimes, they are brown dwarfs, or even other mainstream stars. There’s evidence to suggest that all stars begin life in a binary litter, known as a ‘dense core’ (10). Many of these binary units remain intact, providing the universe with a steady supply of binary stars. Others seem to break apart, flinging the companion object into interstellar space – a rejected runt left to fend for itself within the galactic streams. Spotting binary stars which have relatively similar masses is easy enough. What is less clear is whether tiny companion objects, formed within these dense cores, set up shop within planetary systems, hiding beyond out sight within the outer darkness of their sibling star’s system.
A directly imaged example of such a system has been reported this month, which amply illustrates the potential for such companion objects. A young massive star, forty times more massive than the Sun, has been found to have a companion star in orbit around it whose own mass is about half that of our Sun (11). This means that this young binary pair has a comparative mass ratio of 80:1 (12).
Now, if we were to bring this down to a star system where the main star was the mass of the Sun, its companion object would be be an 1/80 of the mass of the Sun. Such an object would be about 13 Jupiter masses, which is at the lowest mass range from brown dwarfs. The implication of this is that our Sun might have spawned a low mass brown dwarf during its formation. This may be a bi-product of the spinning disk as the primordial disk collapses under its own gravity (11), or, alternatively, the result of a spindling effect due to a passing star within the stellar nursery, as suggested by François Berguerand (4). Conceptually, this could support the notion that many brown dwarfs occur in situ within much more massive star systems, rather than independently in interstellar space. They may not be hanging around their parent star in quite the same way as the companion star GJ 3470b does. Instead, they may be eased out gently into a more loosely bound owbit – perhaps even a comet-like orbit, well beyond the planetary environment.
It’s likely, I suggest, that such disparate binaries are common. In effect, they could have been the precursor for each ‘single’ star we see in the sky. After the initial paring form, the companion is either lost to interstellar space, or becomes loosely bound within the outer reaches of the star system. Perhaps, in some cases, it erodes over times due to its ongoing proximity to the main star blasting away at its atmosphere with ultraviolet radiation, as appears to be the case with certain ‘hot’ Neptune exoplanets, as discussed above (8). In the wild world of exoplanets, anything seems possible.
Written by Andy Lloyd, 15th December 2018
1) R. Allart et al., “Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet,” Science (2018). http://science.sciencemag.org/content/early/2018/12/06/science.aat5879
2) L. Nortmann et al., “Ground-based detection of an extended helium atmosphere in the Saturn-mass exoplanet WASP-69b,” Science (2018), http://science.sciencemag.org/content/early/2018/12/06/science.aat5348
3) University of Exeter Press Release “Helium exoplanet inflated like a balloon, research shows” 6 December 2018 https://www.exeter.ac.uk/news/featurednews/title_695958_en.html
4) “Planet HAT-P-11 b” http://exoplanet.eu/catalog/hat-p-11_b/
5) Instituto de Astrofísica de Canarias Press Release: “Helium signal reveals the comet-like tail of exoplanet WASP-69b for the first time” 6 December 2018 http://www.iac.es/divulgacion.php?op1=16&id=1495&lang=en
6) “Planet WASP-69 b” http://exoplanet.eu/catalog/wasp-69_b/
7) B. Lavie et al. “The long egress of GJ 436b’s giant exosphere” Astronomy & Astrophysics 605, L7 (2017)
8) V. Bourrier et al. “Hubble PanCET: An extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470 b” Astronomy and Astrophysics, 14 December 2018 https://arxiv.org/pdf/1812.05119.pdf
9) Sci News Staff ” Hubble Finds Fast-Evaporating Warm Neptune: GJ 3470b” 14 December 2018 http://www.sci-news.com/astronomy/fast-evaporating-warm-neptune-gj-3470b-06722.html
10) Robert Sanders “New evidence that all stars are born in pairs” 13th June 2017, http://news.berkeley.edu/2017/06/13/new-evidence-that-all-stars-are-born-in-pairs/
11) Leeds University Press Release “A young star caught forming like a planet” 14 December 2018 https://www.leeds.ac.uk/news/article/4343/a_young_star_caught_forming_like_a_planet
12) J. Ilee et al “G11.92-0.61 MM1: A Fragmented Keplerian Disk Surrounding a Proto-O Star”. Astrophysical Journal Letters, Dec 2018, 869(2), arxiv.org/pdf/1811.05267.pdf
13) Andy Lloyd “Our Solar System: An Alternative Birth” 25th September 2015, http://www.andylloyd.org/darkstarblog30.htm
14) Sergio Prostak “Kepler-438b: Red Dwarf May Have Stripped Away Atmosphere of Earth-Like Exoplanet” 18 November 2015 http://www.sci-news.com/astronomy/kepler-438b-red-dwarf-atmosphere-earth-like-exoplanet-03444.html