NASA’S TESS Spots Planet On Incredibly Short 7.7-Hour Stellar Orbit

An international team of astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) has detected a rocky planet, about half the mass of Earth, in an extraordinarily short 7.7-hour orbit around its parent star.

It’s a reminder that the science of extrasolar planet hunting seems to enter bizarro land with each new discovery. Planetary scientists still haven’t figured out how our own tiny Mercury —- which orbits our Sun once every 88 days —- actually formed and evolved. So, this iron-rich ultrashort-period (USP) planet, dubbed GJ 367b should really boggle their minds. 

It’s completely rocky, unlike most previously detected gaseous hot Jupiters on extremely short stellar orbits. As a result, the tiny planet is estimated to have a surface with temperatures of 1500 degrees Celsius, hot enough to melt iron; hardly an Earth 2.0.

As noted in a paper appearing in the journal Science, a team led by the Institute of Planetary Research at the German Aerospace Center (DLR) was able to detect and characterize GJ 367b in detail using both TESS’ high-precision photometry and doppler spectroscopy of its parent star. The latter was accomplished using the Sunacy Radial Velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory’s (ESO) 3.6-meter telescope in Chile.   

TESS observed the planet’s parent red dwarf star, GJ 367, about half the mass of the Sun, for over a period of nearly a month and was able to detect dimming caused by the tiny planet as it transited across the face of its star. Doppler spectroscopy in turn looks for a given star’s gravitational wobble as it is perturbed by an orbiting planet. Combining data from both TESS and HARPS, the team was able to estimate the planet’s mass, its interior structure and its diameter.

"From the precise determination of its radius and mass, GJ 367b is classified as a rocky planet," Kristine Lam, the paper’s lead author and an astronomer at the DLR, said in a statement. "It seems to have similarities to Mercury. This places it among the sub-Earth sized terrestrial planets and brings research one step forward in the search for a 'second Earth'."

Among the lightest of the nearly 5000 extrasolar planets yet detected, GJ 367b has an estimated 9000 km-diameter that is slightly larger than Mars, but is more dense than Earth, the authors note.  

Surprisingly, the planet’s interior structure has an iron-core that makes up more than 85 percent of its radius, not unlike our planet Mercury.  The team estimates that GJ 367b is composed of less than 10 percent water ice and/or a hydrogen or helium gas envelope. 

But the planet's proximity to its star means it is exposed to extremely high levels of radiation, more than 500 times stronger than that experienced by Earth, the DLR reports. Thus, surface temperatureS could reach up to 1500 degrees Celsius – a temperature at which all rocks and metals would be melted, says the DLR.

As for how such ultrashort orbit planets could form or even evolve so near their parent stars?

"We already know a few of these, but their origins are currently unknown," Lam said in a statement. "By measuring the precise fundamental properties of the USP planet, we can get a glimpse of the system's formation and evolution history."

Source: https://www.forbes.com/sites/brucedorminey/2021/12/02/nasas-tess-spots-planet-on-incredibly-short-77-hour-stellar-orbit/?sh=4ddfd5fcb248