Astronomers using the European Space Agency’s Cheops mission mission have caught an exoplanet that seems to be triggering flares of radiation from the star it orbits. These tremendous explosions are blasting away the planet’s wispy atmosphere, causing it to shrink every year.
This is the first-ever evidence for a ‘planet with a death wish’. Though it was theorised to be possible since the nineties, the flares seen in this research are around 100 times more energetic than expected.
This planet’s star makes our Sun look sleepy
Thanks to telescopes like the NASA/ESA/CSA James Webb Space Telescope and NASA’s Transiting Exoplanet Survey Satellite ( TESS ), we already had some clues about this planet and the star it orbits.
The star, named HIP 67522, was known to be just slightly larger and cooler than our own host star, the Sun. But whilst the Sun is a middle-aged 4.5-billion-year-old, HIP 67522 is a fresh-faced 17-million-year-old. It bears two planets. The closer of the two – given the catchy name HIP 67522 b – takes just seven days to whip around its host star.
Because of its youth and size, scientists suspected that star HIP 67522 would churn and spin with lots of energy. This churning and spinning would turn the star into a powerful magnet.
Our much-older Sun has its own smaller and more peaceful magnetic field. From studying the Sun, we already knew that flares of energy can burst from magnetic stars when ‘twisted’ magnetic field lines are suddenly released. This energy can take the form of anything from gentle radio waves to visible light to aggressive gamma rays.
A la carte research with Cheops
Ever since the first exoplanet was discovered in the 1990s, astronomers have pondered whether some of them might be orbiting close enough to disturb their host stars’ magnetic fields. If so, they could be triggering flares.
A team led by Ekaterina Ilin at the Netherlands Institute for Radio Astronomy ( ASTRON ) figured that with our current space telescopes, it was time to investigate this question further.
“We hadn’t seen any systems like HIP 67522 before; when the planet was found it was the youngest planet known to be orbiting its host star in less than 10 days,” says Ekaterina.
The team was using TESS to do a broad sweep of stars that might be flaring because of an interaction with their planets. When TESS turned its eyes to HIP 67522, the team thought they could be on to something. To be sure, they called upon ESA’s sensitive CHaracterising ExOPlanet Satellite, Cheops .
“We quickly requested observing time with Cheops, which can target individual stars on demand, ultra precisely,” says Ekaterina. “With Cheops we saw more flares, taking the total count to 15, almost all coming in our direction as the planet transited in front of the star as seen from Earth.”
Because we are seeing the flares as the planet passes in front of the star, it is very likely that they are being triggered by the planet.
A flaring star is nothing new. Our own Sun regularly releases bursts of energy, which we experience on Earth as ‘ space weather ‘ that causes the auroras and can damage technology. But we’ve only ever seen this energy exchange as a one-way street from star to planet.
Knowing that HIP 67522 b orbits extremely close to its host star, and assuming that the star’s magnetic field is strong, Ekaterina’s team deduced that the clingy HIP 67522 b sits close enough to exert its own magnetic influence on its host star.
They think that the planet gathers energy as it orbits, then redirects that energy as waves along the star’s magnetic field lines, as if whipping a rope. When the wave meets the end of the magnetic field line at the star’s surface, it triggers a massive flare.
It’s the first time we see a planet influencing its host star, overturning our previous assumption that stars behave independently.
And not only is HIP 67522 b triggering flares, but it is also triggering them in its own direction. As a result, the planet experiences six times more radiation than it otherwise would.
A self-imposed downfall
Unsurprisingly, being bombarded with so much high-energy radiation does not bode well for HIP 67522 b. The planet is similar in size to Jupiter but has the density of candy floss, making it one of the wispiest exoplanets ever found.
Over time, the radiation is eroding away the planet’s feathery atmosphere, meaning it is losing mass much faster than expected. In the next 100 million years, it could go from an almost Jupiter-sized planet to a much smaller Neptune-sized planet.
“The planet seems to be triggering particularly energetic flares,” points out Ekaterina. “The waves it sends along the star’s magnetic field lines kick off flares at specific moments. But the energy of the flares is much higher than the energy of the waves. We think that the waves are setting off explosions that are waiting to happen.”
More questions than answers
When HIP 67522 was found, it was the youngest known planet orbiting so close to its host star. Since then, astronomers have spotted a couple of similar systems and there are probably dozens more in the nearby Universe. Ekaterina and her team are keen to take a closer look at these unique systems with TESS, Cheops and other exoplanet missions.
“I have a million questions because this is a completely new phenomenon, so the details are still not clear,” she says.
“There are two things that I think are most important to do now. The first is to follow up in different wavelengths (Cheops covers visible to near-infrared wavelengths) to find out what kind of energy is being released in these flares – for example ultraviolet and X-rays are especially bad news for the exoplanet.
“The second is to find and study other similar star-planet systems; by moving from a single case to a group of 10–100 systems, theoretical astronomers will have something to work with.”
Maximillian Günther, Cheops project scientist at ESA, is excited to see the mission contributing to research in a way that he never thought possible: “Cheops was designed to characterise the sizes and atmospheres of exoplanets, not to look for flares. It’s really beautiful to see the mission contributing to this and other results that go so far beyond what it was envisioned to do.”
Looking further ahead, ESA’s future exoplanet hunter Plato will also study Sun-like stars like HIP 67522. Plato will be able to capture much smaller flares to really give us the detail that we need to better understand what is going on.
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