A multi-planet system in formation ‒ that is what a research lead by PhD student Sebastian Haffert discovered in 2019 for the first time. Using an instrument that was designed for observing galaxies, the team recorded two growing exoplanets. Haffert: ‘It was a lucky coincidence. One of the exoplanets was discovered just when we were looking for an object to test the instrument.’
Exoplanets spotted collecting dust in protoplanetary disc
About 370 light-years away, around a star named PDS 70, two planets are forming: PDS 70b and PDS 70c. They grow by accreting gas and dust from a circumstellar disk surrounding PDS 70. This mass contains a lot of hydrogen. When it falls onto the planets, it gets so hot that it emits deep-red light. Haffert observed this light with the MUSE instrument at the 8.2-m diameter Very Large Telescope in Chile.
‘When the exoplanet PDS 70b was first discovered, the researchers also observed a protoplanetary disk around the star and suspected that the planet was still growing,’ says Haffert. However, those first measurements were not conclusive ‒ just as other observations of possible planet-forming systems.
‘Astronomers have been looking for planets that are growing in protoplanetary disks for a while,’ says Haffert. ‘They’ve seen disks with gaps in them, which could be created by a planet. There are some measurements that hint at the existence of planets in those gaps, but for most instruments it is difficult to distinguish planets from disk material.’
‘Finding a multi-planet system in formation started with a lucky coincidence. One of the exoplanets was discovered just when we were looking for an object to test the instrument on.’
With the MUSE-instrument, Haffert was able to measure the specific light that is emitted by hydrogen that is heated when it falls onto the growing exoplanet. ‘The measurement of this light confirmed that PDS 70b was growing. And to our surprise, we observed another exoplanet, also emitting this light, albeit a bit weaker.’ This probably means the second planet is a bit smaller, causing it to attract less of the surrounding gas.
Haffert: ‘This is the first confident measurement of exoplanets forming in a protoplanetary disk.’ This unique observation was possible thanks to the ability of the MUSE-instrument to measure light spectra with high resolution. ‘Some instruments can only measure the difference between light with wavelengths of 600 and 800 nanometers. With MUSE, we can distinguish 600.1 from 600.2 nanometers.’ That made the observation sensitive enough to see the difference between light from the star, the disk, and the growing exoplanets.
With this observation, we can learn about the way forming planets interact with the surrounding disk. There is still much we don’t know about planet formation. ‘For example, we think that planets don’t necessarily stay where they form,’ says Haffert. ‘Sometimes they move inwards, closer to their parent star, and sometimes they migrate outwards.’
However, one planet-forming system is not enough. Astronomers want more; they can’t base a general theory about planetary system formation on one observed system. ‘We’re currently doing more measurements on other systems with protoplanetary disks’, says Haffert. ‘It’s going to be exciting to see whether they also contain forming planets.’
Haffert will continue his work on observing planetary systems in formation and designing instruments in the US. He was awarded the most prestigious postdoctoral fellowship in astronomy, the NASA Hubble Fellowship. ‘This enables me to work in the US for three years,’ says Haffert. ‘My goal is to develop an instrument to obtain spectral images as good as MUSE’s, but this time designed purely for exoplanet research.’
Even though he grew up in an area with too much light pollution to see the stars, Sebastiaan Haffert (Zoetermeer, 1992) was interested in astronomy from a young age, gaining knowledge from websites and documentaries. He studied and obtained his PhD in Leiden. For his NASA Hubble Fellowship he recently moved to Tuscon, Arizona. ‘Because it is one of the darkest towns in the US, it is very suitable for astronomy research,’ he says. 'The night sky is so beautiful that I might buy my first telescope.’