TESS Discovers First Known Ultrahot Neptune: LTT 9779b | Astronomy

Astronomers have discovered an ultra-short-period Neptune-sized exoplanet orbiting the bright Sun-like star LTT 9779.

An artist’s impression of the ultrahot Neptune LTT 9779b and its parent star, LTT 9779. Image credit: Ricardo Ramirez, Universidad de Chile.

LTT 9779 is a G8-type star located 260 light-years away in the constellation of Sculptor.

Also known as TOI-193, CD-38 15670 and HIC 117883, the star is around 2 billion years old and is metal-rich, having twice the amount of iron in its atmosphere than the Sun.

Named LTT 9779b, the newfound planet orbits so close to its star that its year lasts only 19 hours, and stellar radiation heats the planet to 1,727 degrees Celsius (3,141 degrees Fahrenheit). At these temperatures, heavy elements like iron can be ionized in the atmosphere and molecules disassociated.

The alien world is 4.6 times larger than Earth and 29 times as massive. Its mean density is similar to that of Neptune and its atmosphere makes up around 9% of the total planetary mass.

“LTT 9779 is very rich in metals, with twice as much iron as in the Sun’s atmosphere, which could indicate that the planet was originally a much bigger gas giant, because those types of planets form preferentially near to stars with high iron abundance,” said co-author Dr. Enric Pallé, an astronomer at the Instituto de Astrofísica de Canarias.

The transits of LTT 9779b were detected in data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS).

Follow-up observations with the HARPS spectrograph at ESO’s La Silla Observatory in Chile confirmed the planetary nature of the transits.

“The discovery of LTT 9779b so early in the TESS mission was a complete surprise,” said lead author Professor James Jenkins, an astronomer in the Department of Astronomy at the Universidad de Chile.

“The majority of transit events with periods less than one day turnout to be false-positives, normally background eclipsing binary stars.”

LTT 9779b is located in the so-called ‘Neptunian desert’, a region devoid of planets when we look at the population of planetary masses and sizes.

Models tell us that planets like LTT 9779b should be stripped of their atmospheres through a process called photoevaporation as they move close to their stars.

The large gas giants, on the other hand, have strong gravitational fields that can hold onto their atmospheres, and so we end up with a dearth of planets like Neptune with the shortest orbital periods.

“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,” Professor Jenkins said.

“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.”

“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.”

“LTT 9779b is an intriguing planet, being the first of its kind discovered,” said co-author Dr. Ed Gillen, a researcher in Cavendish Laboratory.

“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.”

The discovery of LTT 9779b is reported in a paper in the journal Nature Astronomy.


J.S. Jenkins et al. An ultrahot Neptune in the Neptune desert. Nat Astron, published online September 21, 2020; doi: 10.1038/s41550-020-1142-z

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