Using a Unique Method, VU Astronomers and International Partners Have Discovered an Exceptional New Planet

Dr Marius Maskoliūnas and Assoc. Prof. Edita Stonkutė.

Scientists from Vilnius University (VU) Faculty of Physics, together with colleagues from Poland and other countries, have identified an exoplanet – a gas giant located far from the Galactic Centre. This is only the third such discovery in the entire history of observations. The discovery is even more exceptional due to the method used – the phenomenon known as microlensing. The results of the observations have been published in one of the most prestigious astronomy journals “Astronomy & Astrophysics”.

Third Such Case in History

“This kind of work requires a lot of expertise, patience, and, frankly, a bit of luck. You have to wait for a long time for the source star and the lensing object to align and then check an enormous amount of data. Ninety per cent of observed stars pulsate for various other reasons, and only a minority of cases show the microlensing effect,” says Dr Marius Maskoliūnas, the head of the Lithuanian research team.

Gravitational microlensing is a rare phenomenon, first predicted by Albert Einstein in the early 20th century. It occurs when a massive body, such as a star or a dark, invisible object, briefly positions itself directly in front of a more distant star. The light from the latter then becomes amplified, as if magnified by an invisible magnifying glass. This temporary light “pulsation” is what astronomers search for while analysing vast amounts of data.

According to the scientist, the collaboration and discovery itself happened almost by chance. It all began during a visit to colleagues at the Astronomical Observatory of the University of Warsaw. One of the method’s enthusiasts, Prof Lukasz Wyrzykowski, suggested preparing a joint Polish-Lithuanian project. His idea was simple – to analyse data from the European Space Agency’s “Gaia” telescope, verify it, and supplement it with ground-based observations. The telescopes at VU’s Molėtai Astronomical Observatory are suitable for this purpose.

The phenomenon that hinted at the location of planet AT2021uey b was first observed in 2021. After scientists carefully verified and analysed the data, they were finally able to determine that it is a gas giant located 3,262 light-years away, with a mass that reaches 1.3 times that of Jupiter. It orbits around a so-called M dwarf – a relatively small and cool star, completing one orbit every 4,170 days. Their unusual size ratio also contributed to the planet’s discovery – detecting an Earth-type planet would have been much more difficult.

As Assoc. Prof. Edita Stonkutė, the leader of the joint Polish-Lithuanian project in Lithuania, notes, no less interesting is where it was detected.

“Most microlensing effects are recorded at the densest part of the galaxy – in its centre and disk. However, we managed to find this microlensing phenomenon quite far from the centre, in the so-called galactic halo. This is only the third planet in observational history to be discovered so far from the Galactic bulge,” states the researcher.

A Promising Search Method

The very first planet orbiting a star was discovered exactly three decades ago, in 1995. Since then, nearly 6,000 more have been confirmed. Nevertheless, this science is still considered relatively young, and astronomers are constantly expanding their knowledge about what planets and their systems might look like.

“When the first planet around a sun-like star was discovered, there was a great surprise that this Jupiter-type planet was so close to its star. As data accumulated, we learned that many types of planetary systems are completely unlike ours – the Solar System. We’ve had to rethink planetary formation models more than once,” explains Assoc. Prof. E. Stonkutė.

The microlensing method is promising because it allows the detection of what is unexpected or even invisible. Dr M. Maskoliūnas reminds us that if we were to add up all the visible mass of the Milky Way, we would obtain, at best, one-tenth of the total mass. In other words, the remaining 90 percent is still invisible to us. Microlensing enables us to unveil this mystery partially.

“What fascinates me about this method is that it can detect those invisible bodies. Other methods work like selective receivers, which, as if with a magnifying glass, focus on a specific cosmic zone that interests you. But in this case, you’re essentially measuring shadows. A very simplified comparison – you’re measuring the duration of some moving object’s shadow. Imagine a bird flying past you. You don’t see the bird itself and don’t know what colour it is – only its shadow. But from it, you can, with some level of probability, determine whether it was a sparrow or a swan and at what distance from us. It’s an incredibly intriguing process,” says the scientist.