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Rare exoplanet photobombs Cheops telescope © European Space Agency

Rare exoplanet photobombs Cheops telescope

Published: 28th June, 2021 at 17:04
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It's the first time an exoplanet with an orbit of more than 100 days has been spotted transiting a star bright enough to be seen with the naked eye.

A rare planet has been found by a team of researchers studying the Nu2 Lupi solar system, located 48 light-years away.


The discovery, published in the journal Nature Astronomy, was made using the European Space Agency’s Cheops satellite.

The planet, called Nu2 Lupi d, is the first time an exoplanet with an orbit of more than 100 days has been spotted transiting a star bright enough to be visible with the naked eye.

The researchers weren’t expecting to see the planet. They were studying two planets orbiting closer to Nu2 Lupi, a Sun-like star in the constellation of Lupus, when the third planet, Nu2 Lupi d, unexpectedly popped into view.

“If placed in our Solar System, Nu2 Lupi d would orbit between Mercury and Venus and so the planet receives a very low amount of star light compared to the majority of discovered exoplanets,” said co-author Dr Thomas Wilson from the University of St Andrews, Scotland.

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The authors established that Nu2 Lupi d is approximately 2.5 times the radius of Earth, but it is around 8.8 times the mass of our planet.

“Transiting systems such as Nu2 Lupi are of paramount importance in our understanding of how planets form and evolve, as we can compare several planets around the same bright star in detail,” said lead author Dr Laetitia Delrez at the University of Liège, Belgium. “We set out to build on previous studies of Nu2 Lupi and observe planets b and c crossing the face of Nu2 Lupi with Cheops, but during a transit of planet c we spotted something amazing: an unexpected transit by planet ‘d’, which lies further out in the system.”

When a planet transits in front of its host star as part of its orbit, it blocks a tiny amount of its light. This creates a valuable opportunity for astronomers to predict the planet’s size, orbit, atmosphere and composition. It’s easiest for researchers to study exoplanets that lie close to their host stars, as their smaller orbits provide more opportunities to study the transits. Planet d takes 107 days to orbit Nu2 Lupi, so the chance of spotting it during a transit was really low, making Cheops’s discovery a surprise.

An infographic of the Nu2 Lupi planetary system © European Space Agency

During this piece of research, the team determined the densities and possible compositions of the planets: b is mostly rock, while c and d (the photobombing planet) contain large amounts of water – far more, in fact, than Earth. However, this water is ice or steam, meaning it is unlikely life could exist there.

“While none of these planets would be habitable, their diversity makes the system even more exciting and a great future prospect for testing how these bodies form and change over time,” said Cheops project scientist Dr Kate Isaak.

The majority of exoplanets with longer orbits have been spotted around stars that are too faint to allow follow-up studies. However, Nu2 Lupi is bright enough to be seen with the naked eye, making it a prime target for future studies.


“Combined with its bright parent star, long orbital period and suitability for follow-up characterisation makes planet d hugely exciting – it is an exceptional object with no known equivalent and is sure to be a golden target for future study,” said co-author Dr David Ehrenreich of the University of Geneva, Switzerland.

Reader Q&A: Could we ever move a planet into a more habitable orbit?

Asked by: Richard Foster-Fletcher

Although it’s theoretically possible to change the orbit of a planet, it’s probably completely impractical.

Moving Mars, for example, to an orbit closer to the Sun would require decreasing its kinetic energy enormously – perhaps by shunting large asteroids into close encounters with it. This would likely take many centuries or millennia to complete, and would require huge amounts of energy. If the aim is to aid in the terraforming of Mars, there would be far cheaper, quicker and more effective ways to do it.

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Alice Lipscombe-SouthwellManaging editor, BBC Science Focus

Alice is the managing editor at BBC Science Focus Magazine. She has a BSc in zoology with marine zoology. Her interests include natural history, wildlife, the outdoors, health and fitness.


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