A unique exoplanet with a mass almost nine times that of Earth has been discovered after it unexpectedly ‘photobombed’ its star 48 light-years away.
The world was found in the constellation of Lupus, also known as Wolf, and is ‘sure to be a golden target for future study’ because there is ‘no known equivalent’, scientists have said.
It 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 to the naked eye.
Discovery: A unique exoplanet has been found after it unexpectedly ‘photobombed’ its star 48 light-years away. An artist’s impression of the Sun-like Nu2 Lupi and its three planets is shown
Orbit comparisons: This graphic shows how long it takes planets B, C and D to circle Nu2 Lupi 48 light-years away, compared to the planets of Venus and Mercury in our solar system
The ‘hugely exciting’ discovery was made by accident while researchers were exploring two other planets in a bright nearby star system.
All three orbit a Sun-like star called Nu2 Lupi, which is one of only three naked-eye stars known to host multiple transiting planets.
Planets ‘b’, ‘c’ and the newly-discovered ‘d’ have masses between those of Earth and Neptune with orbits lasting 11.6, 27.6 and 107.6 days, respectively.
None are habitable, the European Space Agency (ESA) has said, but planet ‘d’ will be a particularly attractive target for both the Hubble Space Telescope and the James Webb Space Telescope, when it launches at the end of this year.
‘Combined with its bright parent star, long orbital period, and suitability for follow-up characterisation, this makes planet d hugely exciting – it is an exceptional object with no known equivalent, and sure to be a golden target for future study,’ said study co-author David Ehrenreich, of the University of Geneva in Switzerland.
Planet d’s orbit would put it between Mercury and Venus in our solar system, he added, while b and c would be inside the orbit of Mercury, which takes 88 days to circle the sun.
Researchers say planet b is mainly rocky but c and d contain far more water than Earth – a quarter of each planet’s mass is made up of water, compared to less than 0.1 per cent for Earth.
This water, however, is not liquid but instead takes the form of high-pressure ice or high-temperature steam.
The discovery of planet d, found to have a mass 8.8 times that of Earth, is one of the first results from ESA’s Cheops (CHaracterising ExOPlanet Satellite).
‘Exciting’: The world was found in the constellation of Lupus, also known as Wolf, and is ‘sure to be a golden target for future study’ because there is ‘no known equivalent’, scientists said
‘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 Kate Isaak.
‘There is also the potential to search for rings or moons in the Nu2 Lupi system, as the exquisite precision and stability of Cheops could allow detection of bodies down to roughly the size of Mars.’
Most long-period transiting exoplanets have so far been discovered around stars that are too faint to allow detailed follow-up observations, meaning little is known about their planets’ properties.
However, Nu2 Lupi is bright enough to be seen by powerful telescopes in space and large observatories on the ground.
Planetary transits – when one celestial object crosses in front of a larger body in space – give researchers an opportunity to study a planet’s atmosphere, orbit, size and interior.
A transiting planet blocks a tiny but detectable proportion of its star’s light as it crosses in front of its star – and it was this drop in light that led Cheops researchers to their discovery.
It is very rare to see long-period exoplanets during transit because they orbit so far from their stars.
‘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 Laetitia Delrez, of the University of Liège in 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.
‘Given its overall properties and orbit, this makes planet d a uniquely favourable target for studying an exoplanet with a mild-temperature atmosphere around a star similar to the Sun.’
The aim of Cheops, an ESA mission developed in partnership with Switzerland, is to help better understand known exoplanets and discover new ones.
The discovery is published in the journal Nature Astronomy.