Is it planet? Is it a star?
Something is growing around a distant star. But what exactly is it growing into?
New observations of a star system in the southern constellation of Chamaeleon, CS Cha, have revealed what seems to be a planet-sized object in the process of forming. The new object, captured in data from the Very Large Telescope (VLT), is one of a small class of such bodies which lie a long way out from their companion stars, making them easier to image but difficult to explain.
This new object, which is about 600 lightyears away, is situated over 30 billion km from CS Cha itself – that’s seven times further out than Neptune is from the Sun. It was first spotted in a survey carried out by the VLT back in 2006, and so by now we can be certain that, despite the great distance involved, it really is a true member of the system, gravitationally bound to its primary. CS Cha is a young double star, and it’s surrounded by what seems to be exactly the kind of disc of gas and dust from which planets form.
So what’s the problem? By the time you get as far out as the companion we see, the density of dust in the disc will be so low that planet formation would take a long time – much longer than the lifetime of the star to date. In fact, the observations show the disc only reaches three-quarters of the way.
The companion doesn’t seem simple, either. The team that studied it, led by astronomers from Leiden in the Netherlands, tried to make sense of what it was seeing using a variety of theoretical models to predict how brightly different planets might shine at different wavelengths, but nothing seems a great fit. There’s something more complicated going on.
A clue comes from observations of polarised light, a technique that reveals whether light has been scattered off dust before reaching the observer. We’d expect some scattering from the disc of dust around the main star, but the signals are distinct, suggesting that the companion itself is surrounded
“Observing the object’s movements over the past 11 years, the team has worked out the eccentricity of its orbit, and it’s very eccentric”
by its own disc or envelope of dust; in other words whatever it is it is still assembling.
I’m reluctant to call it a planet. The best fit to the data is for an object which about 20 times the mass of Jupiter, right around the value that would make something a small brown dwarf star or a large planet. Either way, it’s thrilling to see it still in the midst of assembly. And there’s more.
Observing the object’s movements over the past 11 years, the team has worked out the eccentricity of its orbit, and it’s very eccentric. It has an orbit less circular than Pluto’s! That makes formation from the main disc – where all the dust is in roughly circular orbits – very unlikely, while a history rather like that of a star, which collapses directly from the surrounding gas and dust, much more probable. This unique object may thus be telling us that all of the distant companions imaged over the last 10 years – amongst the most intriguing of planet and planet-like discoveries – formed in this way.
CHRIS LINTOTT was reading… First direct detection of a polarized companion outside of a resolved circumbinary disk around CS Cha by C Ginski et al. Read it online at https://arxiv.org/abs/1805.02261
An infrared image of CS Cha captured by the Very Large Telescope in Chile. The newly discovered companion is highlighted Special polarisation filters make the dust discs visible – the companion seems to have its own dust disc
CHRIS LINTOTT is an astrophysicist and co-presenter of The Sky at Night on BBC TV. He is also the director of the Zooniverse project