If you were to board a plane travelling at 800 kilometres per hour and continue in the direction of the constellation Vulpecula for around 4.6 billion years, you would eventually reach a giant gas planet, found by Lithuanian astronomers working alongside colleagues from Poland and other countries. It is only the second time that researchers from Lithuania have identified a new planet.
“A whole range of factors make this discovery unique,” said Dr Edita Stonkutė, project lead in Lithuania, speaking to LRT.lt.
According to NASA’s exoplanet archive, astronomers around the world have so far identified 5,967 exoplanets – planets beyond our Solar System. One of them is AT2021ueyL b, located 3,392 light years from Earth. The planet was detected through a collaboration between Vilnius University scientists and the University of Warsaw.
Dr Marius Maskoliūnas, one of the study’s authors, said that although the Lithuanian and Polish teams have collaborated since 2019 and already made a number of important findings, this one stood out.
“We’ve discovered many invisible objects before, published joint papers… But this was something new. I immediately said: ‘Oh! This hasn’t been seen before.’”
Spain, for example, has access to major telescopes in the Canary Islands and therefore identifies planets more frequently. “For them it might be routine. For us in Lithuania, it’s a once-in-a-decade kind of event,” explained Dr Stonkutė.
She added that the discovery is particularly significant given the country’s limited resources. “There are only about 30 professional astronomers in Lithuania. For such a small community to contribute to something this rare is truly impressive.”
Most exoplanets discovered using the transit method
“There are thought to be around 200 billion stars in the Milky Way,” says Dr Maskoliūnas, though he notes this is only a statistical estimate and the true number could be slightly higher.
It is believed that almost every star could host planets, but this has yet to be proven.
“At an international conference attended by planetary modellers, I asked a question I often hear myself – does every star have planets? One professor replied: ‘I would be surprised if every star had planets.’ When planets form around a star, they can easily be ejected from the system due to instability, so some stars may have none,” explains Dr Stonkutė.
Exoplanets can be detected using various methods. Only one allows for direct imaging, which requires blocking the star’s light and works only for stars close to Earth. All other methods are indirect. Scientists told LRT.lt that most exoplanets have been found using the transit method.
“Most stars emit light fairly steadily. If a planet passes in front of a star, it temporarily blocks some light, causing a periodic dimming. The period depends on the planet’s orbit,” says Dr Stonkutė. She adds that such observations can take decades:
“If a planet’s orbit is 30 years, you need to observe the star for at least 30 years. That’s why the transit method tends to find planets close to their stars, which orbit more quickly.”
Unique method
The newly discovered planet is notable for being found via gravitational microlensing, a phenomenon that occurs when a massive object – such as a star or a star-planet system – passes in front of a more distant star. The gravity of the foreground object bends and magnifies the light of the background star. If the nearer star has a planet, its gravity can create a brief additional spike in brightness.
“This method allows us to detect planets far from their host stars,” explains Dr Maskoliūnas. He notes that, unlike transits, which repeat periodically and allow repeated observation, microlensing events are usually one-off and unpredictable, making timing crucial.
Dr Stonkutė adds that the newly found planet takes roughly 11 years to orbit its star, meaning conventional transit methods would require a decade of observations to study it in detail.
Since most stars naturally pulsate, causing brightness variations, astronomers must carefully confirm that a signal is caused by microlensing rather than the star itself. This involves additional instruments and archival data analysis.
“Once confirmed, multiple observatories coordinate to study the event and understand what’s happening,” Dr Stonkutė adds.
Gas giant
Once astronomers were confident that they had detected a planet-star system, they used modelling based on available data to infer the likely properties of the newly discovered objects.
“Makiko Ban, the lead author of the study from a Polish observatory, modelled the system and concluded it likely contains a 1.3-Jupiter-mass planet and a star of around half a solar mass. While we cannot directly verify this, the data supports the scenario, and the scientific community has accepted the finding – the paper was peer-reviewed and published in the international journal Astronomy & Astrophysics,” Dr Stonkutė adds.
By its mass, the planet is classified as a gas giant, similar to Jupiter.
“Jupiters are sometimes called ‘failed stars’ – if their mass had been slightly higher, thermonuclear fusion could have begun in their cores, turning them into small stars. But they didn’t acquire enough mass, so they remained planets,” she explains.
Due to the nature of gravitational microlensing, no direct images of the planet or star are obtained.
“This is the essence of the method – we infer the existence of objects from their gravitational effect on light. Some stars barely shine at all, such as neutron stars or white dwarfs, and black holes can even be detected this way, despite emitting no light,” says Dr Maskoliūnas.
Third such planet discovered
Another important detail about the newly discovered planet is its location within the Milky Way galaxy.
The Milky Way consists of a central region containing a black hole, a dense bulge, spiral arms forming the galactic disk, and an enormous surrounding halo.
Roughly 300 of all known exoplanets have been discovered through the effect of gravitational microlensing. The vast majority of them are concentrated in one particular region – near the centre of the Galaxy, close to the central bulge.
“There are more stars there, their density is higher, and when the density of stars is greater, the chances increase that one will pass in front of another and block its light,” explains Dr Maskoliūnas.
What makes the Lithuanian discovery unique is that the distant star whose light was analysed lies in the Galactic halo, while the star-and-planet system causing the lensing effect is located further from the bulge, within the galactic disk.
Using gravitational microlensing, this is only the third such discovery in the world in the entire history of observations.
“It is very difficult to find planets with this method; many more have been found using other techniques,” adds Dr Stonkutė.
The project’s main goal – searching for black holes
As exciting as the discovery of a new planet may be, scientists speaking to LRT.lt admit it was an unexpected bonus. The primary aim of their joint project with colleagues in a neighbouring country is the search for black holes.
“Our project’s goal – together with scientists from Lithuanian and Polish universities – is to analyse invisible objects using the gravitational microlensing effect. In some cases, these objects may turn out to be black holes,” says Dr Stonkutė.
By combining microlensing with data from the European Space Agency’s Gaia space telescope and supplementing it with ground-based observations, researchers can identify massive, otherwise invisible objects.
“If the object detected is far more massive than the Sun – say, two or even 20 times its mass – and it emits no light, then it is likely to be either a neutron star or a black hole,” she explains.
Detecting Earth-mass planets
The same technique can also reveal planets – including so-called free-floating worlds.
“There are stars whose planetary systems break apart. Our own Solar System is stable, but in some cases a planet escapes,” says Dr Maskoliūnas. “These are called ‘free-floating planets’.”
“Such planets emit virtually nothing. They might give off a trace of infrared radiation due to gravity, but the signal is extremely faint. This method, however, is very promising for the search for free-floating planets.”
Overall, Dr Maskoliūnas explains that around 90% of the Milky Way’s mass is invisible to us. “That means 90% of it is hidden somewhere. The mass must exist, but we can only see 10%,” he says.
Gravitational microlensing, he adds, can help reveal these hidden objects and contribute to solving the mystery of the Galaxy’s missing mass.
“Although this method is unlikely to bring about a major breakthrough, it can still broaden our understanding – expanding today’s rather limited picture of the Universe around us.”
Could microlensing also detect planets similar to Earth? Scientists are confident that when it comes to planets with Earth-like mass, the answer is yes.
“Of the roughly 300 exoplanets discovered with this method, most are of similar mass to Jupiter or Neptune. But Earth-type planets have also been found using microlensing. In fact, I believe it was Polish researchers who were the first to detect an Earth-mass planet with this technique,” says Dr Stonkutė.
However, she cautions, mass alone does not determine habitability.
“The planet’s distance from its star is crucial – whether it has a solid surface, liquid water, an energy source and a stable atmosphere.”
Countries conducting fundamental research are richer
Both scientists stress that fundamental astronomy research is not about quick returns.
“People often ask me, what’s the use of what you do? What’s the benefit?” says Dr Maskoliūnas.
“Such research helps us understand what surrounds us, helps map the Milky Way. It’s like needing a map to know where Spain or the Canary Islands are. Similarly, to understand our place in the Universe, we need to understand our own Galaxy.”
“Astronomy isn’t like solar panel manufacturing, where you quickly get financial returns,” he says. “Here, the return is knowledge. But who knows – maybe one day that knowledge will be converted into something more.”
He points to a historic example. “Faraday discovered the law of electromagnetic induction 200 years ago. Could we imagine life today without electricity? Look at how many devices we use that all run on it.”
According to Dr Maskoliūnas, there is also a striking pattern: “All the countries that conduct fundamental scientific research somehow end up living better than those that don’t, or don’t contribute to it.”
Molėtai Observatory – unique in Northern Europe
Dr Stonkutė says that the discovery of the new planet – and more broadly, the preparation of scientific publications in astronomy – could help draw attention to the potential of the Molėtai Observatory, attracting new research projects and investment.
“Lithuania may be small, but our gaze into space is wide,” she explains. “The three telescopes of Vilnius University’s Faculty of Physics at Molėtai Observatory, equipped with top-level scientific instruments, allow us to observe the northern hemisphere skies that telescopes in, for example, the Canary Islands or Chile cannot see. And although we have clear nights for only about a quarter of the year, we dedicate every one of them to science.”
