This Mysterious Planet Could Be Transforming Into a Water World

Only 138 light-years away from Earth, an enigmatic world may be undergoing a transformation.

An study of the extraterrestrial HD-207496b shows that it has a gaseous atmosphere, a global ocean, or a combination of both, and that it may be contracting to become a super-Earth. The planet is 6.1 and 2.25 times the mass and radius of Earth, respectively.

This could assist scientists in explaining a puzzle in planetary detections: the difference in mass between gaseous planets smaller than Neptune and rocky planets larger than Earth. To describe the environment of the mysterious exoplanet, however, requires a closer examination.

There are many exoplanets that are very distinct in our galaxy, which is varied. Approximately 5,300 extrasolar planets have been found and verified by astronomers as of this writing, and nearly twice as many more are possibilities.

Scientists can use this data to perform statistical studies to identify patterns in planetary systems. And one intriguing finding is that there is a striking dearth of exoplanets with rotations shorter than about 100 days and between 1.5 and 2 times the mass of Earth.

The tiny planet radius region is where this is located. Earth, Venus, and Mars-like stony planets, which we refer to as super-Earths, are typically found beneath it.

We refer to the planet's surrounding planets as mini-Neptunes because they resemble small Neptunes and have dense atmospheres.

Although the causes of the valley are not fully understood, an increasing mass of data is starting to point in the direction of close proximity to the host star as a possible explanation. An exoplanet may not have enough mass to maintain a gravity hold on its atmosphere below a certain crucial level (the gas is evaporated by the star's radiation).

We've found a few planets that provide hints about this process, and researchers are searching for more using the High Accuracy Radial Velocity Planet Searcher (HARPS) on the 3.6-meter telescope of the European Southern Observatory at La Silla Observatory in Chile. They are doing this by checking out candidates found by NASA's space-based exoplanet-hunting telescope TESS.

This is what drew a multinational crew to HD-207496b under the direction of astronomer Susana Barros of the University of Porto in Portugal.

With its delicate sensors calibrated to the incredibly faint dips in starlight that could be proof of an orbiting exoplanet crossing, or transiting, between us and the star, TESS scans a region of the heavens in search of exoplanets.

Astronomers can readily assume the existence of an orbiting entity and calculate its period if these transits occur frequently.

Astronomers can determine the radius of the orbiting mass by measuring the depth of the transit dips, or how much sunlight is obstructed, if the star's brightness is known.

HARPS finds a different measure. An exoplanet has its own gravity force as it revolves around a star. Technically speaking, the exoplanet does not revolve around the star; instead, it revolves around their shared center of mass, or barycenter. Stars wiggle around on the spot minutely because they are so much more massive than the planets they orbit.

HARPS is able to detect this. The wavelength of the star's radiation varies as it wiggles toward and away from us, contracting as it gets closer and expanding as it gets farther. Astronomers can also determine the mass of the extraterrestrial since it affects how much the star travels.

A planet's density can be determined once you have knowledge of its mass and radius. Here things start to get really intriguing because density can be used to determine the composition of the extraterrestrial.

They turned to HARPS for a closer look when TESS discovered an exoplanet near the radius valley with a radius 2.25 times that of Earth and a trajectory of 6.44 days around an orange dwarf star called HD-207496. HD-207496b has a mass that is roughly 6.1 times that of the Earth, according to the HARPS measurements.

This indicates that the world has a mass of approximately 3.27 grams per cubic centimeter. That suggests that HD-207496b's makeup is not completely rocky because it is significantly less dense than Earth's density of 5.51 grams per cubic centimeter. In order to determine what the universe is composed of, the experts used modeling.

The experts conclude in their article that HD-207496b has a density lower than Earth, so they predict that it contains a sizable quantity of gas and/or water. We deduce that the planet has either a water-rich envelope, a gas-rich envelope, or a combination of both from interior structure simulations of the planet.

A hydrogen- and helium-rich atmosphere on an exoplanet is only transient, according to evaporation simulations, as the star will strip the planet of all its atmosphere in 520 million years. It's also conceivable that HD-207496b is already a barren ocean planet because the atmosphere has already vanished.

The planet is expected to have both water and a H/He envelope and fall somewhere in the middle of these two scenarios, according to the experts.

Around 520 million years old, the star HD-207496 is a youthful one. If HD-207496b does indeed turn into a naked super-Earth, it will be a unique chance to investigate one of these exoplanets in its formative years before that happens.

The real nature of this enigmatic world—as well as its final fate—should be revealed by follow-up research to describe the atmosphere, if there is one.

The research has been accepted in Astronomy & Astrophysics and is available on arXiv.