‘Hell planet’ orbits its star every 17.5 hours

An artist’s impression of the planet 55 Cnc e (smaller, dark orange circle) blocking the light from its rotating host star. (Credit: Maggie Chiang/Simons Foundation)

New technology has helped astronomers follow the fiery trail of the so-called “hell planet.”

It’s an exoplanet located 40 light years from Earth and nicknamed for its extremely close orbit to its sun.

Debra Fischer, a professor of astronomy at Yale University, developed the instrument that enabled the work, which is housed at the Lowell Observatory’s Lowell Discovery Telescope in Arizona.

The EXtreme PREcision Spectrometer (EXPRES), has captured ultra-precise measurements of the starlight from the distant planet’s sun. Astronomers have now analyzed those measurements to determine the orbit of planet 55 Cnc e.

55 Cnc e, which was discovered in 2004, is so hot (roughly 3,600 degrees Fahrenheit) that its surface is an ocean of lava.

“While the Earth completes one orbit around our sun in 365 days, the planet studied here orbits once every 17.5 hours, hugging its host star, 55 Cnc,” says Fischer senior author of the study in Nature Astronomy.

“Astronomers expect that this planet formed much farther away and then spiraled into its current orbit,” Fischer says. “That journey could have kicked the planet out of the equatorial plane of the star, but this result shows the planet held on tight.”

Lily Zhao, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics in New York and a former Yale graduate student who worked with Fischer on EXPRES, is the study’s lead author.

“This result uses data from EXPRES to determine the spin-orbit alignment of 55 Cnc e, which is an ultra-short period super-Earth exoplanet in a five-planet system,” Zhao says. “Our measurement places interesting constraints on the dynamical history of this system and how planets migrate so close-in to their host stars. This is the smallest such measurement to date.”

The primary mission of EXPRES is finding Earth-like planets based on the slight gravitational influence they have on their stars.

EXPRES does this by using the Doppler effect. When a light source moves toward you, the wavelength of the light you see is shorter and more blue. When the light source moves away, the wavelength is wider and more red. EXPRES was designed to measure shifts in wavelength with unprecedented precision.

In the new study, the researchers learned that 55 Cnc e orbits 55 Cnc along the star’s equator—unlike the other four planets in the system, which are on much different orbital paths.

This implies that the planet may have formed in a relatively cooler orbit farther out and slowly fell toward its sun over time. As the planet moved closer, the stronger gravitational pull from the sun’s thicker, equatorial middle altered the planet’s orbit.

The findings may help researchers better understand how planets form and move over time, Zhao and Fischer say. Such information is critical to finding out how common it is for an Earth-like environment to exist elsewhere in the universe.

“Our precision with EXPRES today is more than 1,000 times better than what we had 25 years ago when I started working as a planet hunter,” Fischer says. “Improving measurement precision was the primary goal of my career because it allows us to detect smaller planets as we search for Earth analogs.”

Additional coauthors are from the Lowell Observatory, San Francisco State University, MIT, the University of Maryland, NASA’s Goddard Space Flight Center, and Yale.

Source: Yale University