New research sheds light on how the ‘hell planet’ got so fiendishly hot and how other worlds might become too toasty for life. That rocky world, 55 Cnc e (called “Janssen”), orbits its star so close that a year lasts just 18 hours, its surface is a giant lava ocean, and its interior may be full of diamonds.
The new insights come thanks to a new tool called EXPRES that captured ultra-precise measurements of starlight shining from Janssen’s sun, known as Copernicus or 55 Cnc. The light measurements changed very slightly as Janssen moved between Earth and the star (an effect similar to our moon blocking the sun during a solar eclipse).
By analyzing these measurements, the astronomers discovered that Janssen orbits Copernicus along the star’s equator, unlike the other Copernican planets, which are on such different orbital paths that they don’t even cross between the star and Earth, researchers reported Dec. 8 in Nature Astronomy.
The implication is that Janssen probably formed in a relatively cooler orbit further away and slowly fell toward Copernicus over time. As Janssen approached, Copernicus’ gravitational pull altered the planet’s orbit.
“We’ve learned how this multiplanetary system, one of the systems with the most planets we’ve found, got to its current state,” says lead study author Lily Zhao, a researcher at the Center for the Flatiron Institute. Computational Astrophysics (CCA) in New York City.
Even in its original orbit, the planet “was probably so hot that nothing we know could survive on the surface,” Zhao says. Still, the new findings could help scientists better understand how planets form and move over time. This information is critical to figuring out how common Earth-like environments are in the universe, and thus how abundant extraterrestrial life might be.
Our solar system, after all, is the only place in the cosmos where we know life exists. It’s also flat as a pancake: all the planets orbit within a few degrees of each other, having formed from the same disk of gas and dust. When exoplanet-hunting missions began to discover worlds around distant stars, they found many planets that did not orbit their host stars in a flat plane. This raised the question of whether our pancake-like solar system really is an oddity.
The Copernican planetary system, located 40 light-years from Earth, is of particular interest given its study and complexity: five exoplanets orbit a main-sequence star (the most common star category) in a binary pair with a red dwarf star. In fact, Janssen was the first “super-Earth” discovered around a main sequence star. Although Janssen has a similar density to Earth and is probably rocky, it is about eight times more massive and twice as wide.
After its discovery and confirmation, Janssen became the first known example of an ultra-short-period planet. Janssen’s orbit has a minimum radius of approximately 2 million kilometers. (For comparison, Mercury’s is 46 million kilometers, and Earth’s is about 147 million.) Janssen’s orbit is so tight around Copernicus that at first some astronomers doubted its existence.
Determining Janssen’s path around Copernicus could reveal a lot about the planet’s history, but making such measurements is incredibly difficult. Astronomers have studied Janssen by measuring the drop in Copernicus’ brightness every time the planet passes between the star and Earth.
This method does not tell you which direction the planet is moving. To find out, astronomers take advantage of the same Doppler effect used in speed cameras. When a light source moves toward you, the wavelength of the light you see is shorter (and therefore bluer). As it moves away, the frequency shifts wider and the light is redder.
When Copernicus rotates, half of the star rotates towards us and the other half moves away. This means that half of the star is slightly bluer and the other half is slightly redder (and the space in between is not shifted). So astronomers can track Janssen’s orbit by measuring when it blocks light from the redder side, the bluer side, and the unaltered middle section.
The resulting difference in starlight, however, is almost immeasurably small. Teams had tried this before, but could not accurately determine the planet’s orbital path. The new research breakthrough comes from the EXtreme PREcision Spectrometer (EXPRES) at the Lowell Discovery Telescope at the Lowell Observatory in Arizona. True to its name, the spectrometer offered the precision needed to notice the small red and blue changes in light.
The FAST measurements revealed that Janssen’s orbit is roughly aligned with the Copernican equator, a path that makes Janssen unique among its siblings.
Previous research suggests that the red dwarf’s close orbit caused the planets to misalign relative to Copernicus. In the new study, the researchers propose that interactions between celestial bodies moved Janssen to his current hellish location. As Janssen approached Copernicus, the star’s gravity became more and more dominant. Because Copernicus is spinning, centrifugal force caused its central section to bulge outward slightly and its top and bottom to flatten. This asymmetry affected the gravity that Janssen felt, causing the planet to line up with the star’s thicker equator.
With Janssen’s story illuminated, Zhao and his colleagues plan to study other planetary systems. “We hope to find planetary systems similar to our own,” he says, “and to better understand the systems we know.”
Zhao co-authored the new paper with Vedad Kunovac and Joe Llama of the Lowell Observatory; John Brewer of San Francisco State University; Sarah Millholland of the Massachusetts Institute of Technology; Christina Hedges of the University of Maryland and NASA’s Goddard Space Flight Center; and Andrew Szymkowiak, Rachael Roettenbacher, Samuel Cabot, Sam Weiss, and Debra Fischer of Yale University.
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