Is it planet? Is it a star?

Some­thing is grow­ing around a dis­tant star. But what ex­actly is it grow­ing into?

Sky at Night Magazine - - BULLETIN -

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

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