An Exo-Neptune Beat the Odds and Kept its Atmosphere

As planet-hunting scientists find increasingly and increasingly planets, they’ve encountered some puzzles. One of them concerns the lack of Neptune-size worlds orbiting tropical to their stars. Astronomers think that these planets aren’t massive unbearable to retain their atmospheres in the squatter of their stars’ powerful radiation, which strips it away.

But at least one of these planets has retained its atmosphere. How?

Astronomers have a name for this lack of Neptune-size planets near stars. They undeniability it the Neptunian Desert, or sometimes the ‘evaporation desert.’

The term has only a wholesale definition, and is usually described as the region so tropical to a star that the orbital period is between only two to four days. It’s moreover specified by the lack of Neptune-size planets with well-nigh one-tenth of Jupiter’s mass. Typically, planets lose their atmospheres when they migrate this tropical to stars and are reduced to only rocky cores, mere remnants of their once puffy selves.

One planet that has retained its undercurrent in the Neptunian Desert is LTT 9779 b. It orbits a G-type star well-nigh 260 light years away. It has 29 Earth masses, and has retained its undercurrent despite stuff only 0.01679 AU from its star and taking only 0.8 of a day to well-constructed an orbit. In this situation, the star’s overwhelming radiation should have simply removed the planet’s atmosphere. Why hasn’t it?

New research set out to wordplay that question. Its title is “Survival in the Neptune desert: LTT 9779 b kept its undercurrent thanks to an unusually X-ray faint host star.” It’ll be published in the Monthly Notices of the Royal Astronomical Society. The lead tragedian is Jorge Fernandez Fernandez, a PhD student in the Astronomy and Astrophysics group at the University of Warwick.

Photoevaporation is a well-understood miracle and it’s linked to stellar rotation. All stars rotate, and when they rotate rapidly, they generate powerful magnetic fields which in turn momentum powerful electromagnetic energy in the form of x-rays and UV radiation. When these energetic photons strike molecules in a planet’s atmosphere, they push the molecules into space. Only a planet’s gravity can counteract it, which explains why there are so many massive hot Jupiters, and scrutinizingly no planets in the Neptunian Desert.

LTT 9779 b is the only known Neptune type planet with an orbital period under one day that has a significant hydrogen/helium atmosphere. In order for the planet to hang onto its undercurrent in such tropical proximity to its star, something unusual must be happening. “If the Neptune desert is the result of X-ray/EUV-driven photoevaporation, it is surprising that the undercurrent of LTT 9779 b survived the intense bombardment of upper energy photons from its young host star,” the authors write.

The wordplay must lie in the star itself, since there’s nothing a planet this size can do to shield itself. It’s directly in the path of its star’s powerful output with nothing to shield it. To examine the star increasingly closely, the researchers overdue this study used XMM-Newton, the ESA’s X-ray observatory launched in 1999.

The spacecraft is moreover tabbed the Upper Throughput X-ray Spectroscopy Mission X-ray Multi-Mirror Mission. Its mission is to investigate interstellar x-ray sources, and though it was launched with a planned 10-year mission, it’s still going without scrutinizingly 24 years. XMM-Newton data is at the heart of this research.

A star’s X-ray emissions are strengthened by its spin. A upper rate of spin generates stronger magnetic fields, which ways stronger X-ray emissions, and slower spin ways weaker emissions. LT 9779’s rotational velocity is well-nigh 1.06 km/s, and it takes well-nigh 45 days to well-constructed one revolution, though the data supporting that is a little weak. Compare that to the Sun’s quicker rotational velocity of 1.997 km/s. That’s scrutinizingly twice as fast, and the Sun is on the slow side compared to most stars. Hot stars can often rotate faster than 100 km/s. From this perspective, LT 9779 is rotating at a snail’s pace.

Age is flipside factor in a star’s x-ray emissions, and the researchers compared its emissions with its age. “We observed LTT 9779 with XMM-Newton and measured an upper limit for its X-ray luminosity that is a factor of fifteen lower than expected for its age,” the paper states.

The researchers moreover modelled the planet’s internal structure and how that unauthentic its mass-loss history. They modelled the planet’s radius, envelope mass fraction, and mass loss rate under two variegated XUV histories. One had an expected stellar emission history and one had a faint stellar emission history.

They found that “… the survival of its undercurrent to the present day is resulting with an unusually faint XUV irradiation history that matches both the X-ray and rotation velocity measurements.”

So what happened in this system that one of its planets has survived in the desert?

Previous research suggested that this unusual scenario is due to late inward-migration by the planet, followed by what’s tabbed Roche-lobe overflow. Roche-lobe overflow typically occurs in binary star systems, where one star can’t hold onto all its mass and the uneaten material forms an toting disk virtually the second star. But in this specimen there’s a single star drawing material from a planet, and equal to this earlier research, the planet started out as a Jupiter-mass planet that lost much of its material to the star, leaving the Neptune-size LTT 9779 b behind.

But that subtitle doesn’t hold up, equal to this work. These researchers arrived at a variegated conclusion that doesn’t involve migration.

“We conclude that LTT 9779 most-likely worked as an anomalously slowly rotating star, and that its close-in Neptune-sized planet LTT 9779 b was thus worldly-wise to survive in the Neptune desert to the present day due to unusually low X-ray irradiation,” they write in their conclusion.

More supporting vestige comes from the planet’s undercurrent itself. It has extremely upper metallicity, and heavier molecules are increasingly difficult to strip yonder than lighter ones. It moreover has a upper albedo that reflects some of the star’s radiation. That can only have helped LTT 9770 b retain its atmosphere.

This research supports the idea that photoevaporation is overdue the Neptune Desert. It would be an incredible coincidence if the only planet in the Neptune Desert that retained its undercurrent is virtually a very slowly-rotating star with weak emissions, and the weak emissions had nothing to do with it. That would stretch credulity.

“Finally, our conclusion that the only known planet deep in the Neptunian desert with a gasesous envelope is moreover unusual in having an X-ray faint star, strongly supports the suggestion that the primary
origin of the Neptunian desert is X-ray driven photoeveporation.”