Exoplanet in the hot-Neptune desert is the first of its kind

Planets and moons orbiting a star.
Enlarge / Artist’s conception of a gas giant planetary system. Unlike TOI-849b, this one still has its enormous gassy envelope.

In 2018, scientists monitoring TESS—the Transiting Exoplanet Survey Satellite—discovered a giant planet in close orbit around the star TOI-849, roughly 735 light years away. Nature recently published a paper from scientists at the Centre for Exoplanets and Habitability that describes this planet in detail.

TOI-849b is roughly 40 times the mass of Earth, which would normally indicate a gas giant, weighing in somewhere between Saturn and Neptune. Yet its radius is smaller than Neptune’s, although it has more than double Neptune’s mass. Since it’s hard to see how that can happen in a gaseous world, the scientists suggest that the planet may once have been a gas giant, stripped now nearly to its solid core. The alternative is that it might have formed in a highly unusual way that allowed it to avoid accumulating much gas in the first place.

TOI-849b’s host star is a late G-class yellow dwarf, a little cooler and approximately 2 billion years older than our Sun, with about nine-tenths its mass and radius. TOI-849b is very close to its host—its orbital period is a little over 18 hours. That means the farthest distance from its star is only 0.02 Astronomical Units (the typical distance between Earth and our Sun). For reference, our own Mercury orbits the Sun nearly 200 times more distantly, at almost 0.4 AU.

The barren area in the vicinity of TOI-849b's combination of mass and orbital period is known as the "hot-Neptune desert" due to the unlikely formation of planets with this combination of characteristics.
The barren area in the vicinity of TOI-849b’s combination of mass and orbital period is known as the “hot-Neptune desert” due to the unlikely formation of planets with this combination of characteristics.

The radius, mass, and period of TOI-849b place it firmly in a zone that’s called the “hot-Neptune desert”—an orbital region where the physics are such that large planets appear to very rarely form. The rarity of planets with large mass and close orbital period is likely due to a combination of two processes driven by the star: photoevaporation and tidal disruption. Photoevaporation is the ionization of gases and particles from intense radiation, causing their dispersion; tidal force is the difference between gravitational force at the near and far sides of a mass, which causes a stretching effect.

This radiotelescope image of the young star HL Tauri clearly shows the protoplanetary disc gaps caused as planets form. If other planets form in these gaps, they could end up gas poor.
Enlarge / This radiotelescope image of the young star HL Tauri clearly shows the protoplanetary disc gaps caused as planets form. If other planets form in these gaps, they could end up gas poor.

Factors leading to TOI-849b’s own largely missing gas envelope could include anything that added enough energy to dissipate its atmosphere back into space. That means any combination of planetary collision, tidal disruption, and photoevaporation. Alternatively, it could have just formed at a time when there was very little gas present, perhaps due to a planetary formation relatively late in its star’s life or from a gap in the protoplanetary disk largely devoid of gases.

Although we don’t yet know the details of TOI-849b’s unusual formation and history, its discovery offers astrophysicists fascinating clues to the mechanics of planetary formation and new insights to the likely composition of gas giant cores.

Nature, 2020. DOI: 10.1038/s41586-020-2421-7  (About DOIs).