Planet hunting at Vega
There are signs of exoplanets around one of the sky’s most famous stars, but they remain elusive
Vega is the second brightest star in the northern hemisphere, and was well known to ancient Babylonian, Assyrian and Chinese stargazers. It was the northern pole star when the last Ice Age was thawing 14,000 years ago, before precession of Earth’s axis rolled round to align with Polaris instead. Vega was also the first star ever to be photographed – by astronomer William Cranch Bond and photography pioneer John Adams Whipple using Harvard College Observatory’s 15-inch Great Refractor – in July 1850, as well as one of the first to have its distance estimated by parallax. So for many reasons, Vega is considered to be one of the most important stars in the history of astronomy. It was also significant in Carl Sagan’s novel Contact; being the star from which humanity received an unambiguously artificial radio transmission. But does Vega actually possess any planets?
What we do know is that Vega is pretty young – about 450 million years old – and is still surrounded by a swirling disc of dust; we can see an ‘infrared excess’ from Vega emitted by these warm grains. This is the raw material from which new planets form around a young star. Vega’s dusty disc just so happens to be orientated almost perfectly face-on to us, and astronomers have detected an inner, warmer ring of dust at around 14 AU and an outer, cooler ring between about 50 and 140 AU. Crucially, there is a large gap between these dust rings which strongly hints at the existence of several newly-formed planets that have swept clear this orbital space.
Frustratingly, though, because the Vega system is face-on to us, the most successful methods for exoplanet detection – radial velocity or transit – won’t work. So instead, Tiffany Meshkat at the California Institute of Technology, Pasadena, and her colleagues have tried to directly image any large planets orbiting this bright, young star. They used an instrument installed at the Palomar Observatory to observe Vega’s dusty disc whilst blocking the central brilliance of the star itself with a coronagraph. And what did they find..? Absolutely nothing. Zip all. But this certainly isn’t an anti-climax. And I think this study very neatly demonstrates a key principle in science: often, not finding something can be almost as informative as actually discovering it. The fact of the non-detection, coupled with your understanding of the sensitivity of your equipment, allows you to place tighter and tighter constraints on what could be present but yet remain undetected. In this case, that turns out to be the maximum size of any planets orbiting Vega: if they were orbiting within the warm dust belt and were any larger than 20 times the mass of Jupiter, Meshkat and her team would have spotted them.
Something is clearly sculpting the debris disc in the Vega system, and we now know a lot more about how big these planets could be, even without actually having discovered them. With the launch of the James Webb Space Telescope in 2021, and its much greater sensitivity, we will be able to spot any planets in the Vega system right down to the mass of Saturn (which is about a third the mass of Jupiter). Vega is unusually bright and nearby, and so this would offer us a spectacular opportunity to study a whole new solar system still in the making.
LEWIS DARTNELL was reading… A deep search for planets in the inner 15 AU around Vega by Tiffany Meshkat. Read it online at arxiv.org/abs/1809.06941
“A key principle in science is that often not finding something can be almost as informative as discovering it."