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Cambridge astronomers help discover new class of planet known as an ‘ultrahot Neptune’

A new class of planet - known as an ‘ultrahot Neptune’ - has been discovered by an international team of astronomers , including University of Cambridge researchers.

Found orbiting the nearby star LTT 9779, which lies 260 light years away in a two billion-year-old system, the planet is very peculiar indeed.

An artist's impression of LTT 9779b, an 'ultrahot Neptune' planet. Pictuire: Ricardo Ramirez, Universidad de Chile
An artist's impression of LTT 9779b, an 'ultrahot Neptune' planet. Pictuire: Ricardo Ramirez, Universidad de Chile

It is so close to its star that its year lasts only 19 hours, and the stellar radiation heats the planet to more than 1,700 degrees Celsius.

The puzzle for astronomers is why it has been able to keep its atmosphere for so long, given this intense irradiation.

“LTT 9779b is an intriguing planet, being the first of its kind discovered,” said Dr Ed Gillen, from Cambridge’s Cavendish Laboratory, who co-authors a paper on it in Nature Astronomy.

“It is particularly exciting because of its peculiarity: how did this planet come to arrive on such a short period orbit and why does it still possess an atmosphere? Fortunately, the planetary system is located nearby so we can study it in detail, which promises new insights into how such planets come to be and what they are made of.”

It is thought it may originally have been a much larger gas giant, since these form close to stars with the highest iron abundances, like LTT 9779.

Professor James Jenkins, from the Department of Astronomy at the Universidad de Chile, who led the team, said: “Planetary structure models tell us that the planet is a giant core dominated world, but crucially, there should exist two to three Earth-masses of atmospheric gas. But if the star is so old, why does any atmosphere exist at all? Well, if LTT 9779b started life as a gas giant, then a process called Roche Lobe Overflow could have transferred significant amounts of the atmospheric gas onto the star.”

Under this process, a planet comes so close to its star that the star’s stronger gravity captures the outer layers of the planet, causing it lose mass.

“It could also be that LTT 9779b arrived at its current orbit quite late in the day, and so hasn’t had time to be stripped of the atmosphere. Collisions with other planets in the system could have thrown it inwards towards the star. Indeed, since it is such a unique and rare world, more exotic scenarios may be plausible,” said Prof Jenkins.

The planet was initially found using the Transiting Exoplanet Survey Satellite (TESS), which measures light changes as the shadow of orbiting planets pass over stars.

Observations at the ESO la Silla Observatory in northern Chile using the HARPS spectrograph, which uses the Doppler Wobble method to measure planet masses and orbital characteristics, confirmed the find.

Cambridge’s astronomy department, part of the Next-Generation Transit Survey (NGTS), carried out follow-up observations.

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