Just over 300 light-years away is a star that’s a lot like a very young version of our Sun, with multiple exoplanets orbiting it. That’s an interesting find in itself. But what makes the system truly dazzling is that it just became the first of its kind to be directly imaged, planets and all. On the night of 16 February 2020, astronomers using the Very Large Telescope in Chile were able to obtain direct observations of two enormous exoplanets on extremely large orbits around the star named TYC 8998-760-1.
Directly imaging exoplanets is challenging, to say the least. They are very dim compared to their host stars, and very far away from us. Most of the over 4,000 exoplanets confirmed to date have only been detected via indirect means – such as faint, regular dips in the star’s light as the exoplanet passes in front of it, or a slight wobble in the star’s position due to the exoplanet’s gravity. Because these signals are easier to detect when the planet is very large and very close to the star, the majority of confirmed exoplanets are large and on close orbits. But exoplanets on very close orbits are difficult to image directly, because they tend to be vastly outshone by their host stars; and distantly orbiting planets in older systems are too cool for infrared detection.
To date, only a few tens of exoplanets have been directly imaged, and only two other multi-planet systems – both around stars very different from the Sun.
But last year, using direct imaging, a team of astronomers led by Alexander Bohn of Leiden University in the Netherlands found an unusual planet orbiting TYC 8998-760-1.It was a gas giant around 14 times the mass of Jupiter, orbiting the star at a distance of around 160 astronomical units. To put that in perspective, Pluto orbits the Sun at an average distance of 39 astronomical units.
So Bohn and his colleagues decided to take a closer look, using the Very Large Telescope’s exoplanet-imaging SPHERE instrument. They took several observations over the last year, and added them to data dating back to 2017.When all the data were put together, they held a surprise. Clear and bright, there was the exoplanet they expected to see, TYC 8998-760-1 b. But, at a much greater distance of 320 astronomical units, the astronomers found another bright dot. Careful analysis and comparison of images taken at different times revealed this wasn’t a star or glitch, but a second, smaller exoplanet, clocking in at about six times the mass of Jupiter. It’s been named TYC 8998-760-1 c. Such images aren’t just wonderful achievements of science and technology, they can also help us to better understand planetary systems. For one thing, TYC 8998-760-1 is young, only 16.7 million years old. Studying the exoplanets that orbit young Sun-like stars can give us valuable insight into the formation of planetary systems like our own. The orbital distance the team detected is already quite interesting, because one model of planetary system formation posits that giant planets form at a distance before migrating inwards towards their host star.
For another, direct images of exoplanets can help us in the search for habitability. Detailed spectroscopic images – breaking down the spectrum of light reflected off an exoplanet – can reveal the presence of an atmosphere, and even the composition of that atmosphere. Photometry, or studying the exoplanets’ brightness and variability thereof, can reveal information about cloud cover and abundance. We’re not quite at that stage yet, but future instruments, such as the James Webb Space Telescope, and the European Southern Observatory’s ground based Extremely Large Telescope, ought to be sensitive enough to start making such detections.
And they might even be able to find smaller, closer planets in this system that SPHERE may have missed.