Evaporating exoplanet WASP-69b solves two planetary puzzles | by Ethan Siegel | Starts With A Bang! | Jan, 2024

by TexasDigitalMagazine.com

Around the star WASP-69, a “hot Jupiter” exoplanet has its outer layers of atmosphere photoevaporated away, creating a comet-like tail whose extent and mass are being measured for the first time. (Credit: W. M. Keck Observatory/Adam Makarenko)

Planets can be Earth-like or Neptune-like, but only rarely are in between. This hot, Saturn-like planet hints at a solution to this puzzle.

It was only a little over 30 years ago, in the early 1990s, that humanity detected our first planets in orbit around stars other than the Sun: the exoplanets. The earliest ones discovered were a bit of a surprise: they were all massive, in tight orbits around their parent stars, and extremely hot: a class known as hot Jupiters. Since that time, we’ve discovered more than 5000 exoplanets, ranging from sub-Earth sized all the way up to super-Jupiters, with a huge variety found in between. However, two puzzles have arisen:

  • the fact that there are “hot Earths” and “hot Jupiters,” but no “hot Neptunes” in between them,
  • and the fact that there are plenty of Earth-like planets up to about 140% the radius of Earth, and plenty of Neptune-like planets down to about half the size of Neptune (about 200% the radius of Earth), but preciously few planets in that in-between range: a puzzle known as the radius gap.

Although there are suspects in the quest to solve these cosmic puzzles, the direct supporting evidence has been elusive up until now. However, a new paper that investigates the remarkable properties of evaporating exoplanet WASP-69b may provide the key clue that leads to a solution for both. Here’s how.

The idea of the radial velocity method is that if a star has an unseen, massive companion, whether an exoplanet or a black hole, observing its motion and position over time, if possible, should reveal the companion and its properties. This remains true, even if there’s no detectable light emitted from the companion itself. (Credit: E. Pécontal)

The first successful methods for finding exoplanets involved measuring the light from a parent star very exquisitely. If there are planets orbiting the star in question, the star isn’t only going to gravitationally pull on that planet, but the planet will gravitationally pull on the star, causing the star to move in an elliptical pattern around the mutual center-of-mass of the star-planet system. That causes the star to appear to “wobble” with respect to us, as it will periodically move towards-and-away from us leading to its light redshifting and blueshifting in a periodic fashion. This detection method, known as either the radial…

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