Extrasolar planet researchers obsess night and day about detecting habitable solar systems and planets. But the vast majority of planets in our galaxy are hardly habitable and many are just downright bizarre.
Case in point is 55 Cancri e, a crazy, super earth-sized planet that orbits its parent red dwarf star in only 17 hours. Located some 40 light years away in the constellation of Cancer, the planet’s surface can get hot enough to melt iron and is known to harbor a global lava ocean.
But until recently it was assumed that 55 Cancri e would be rotationally, tidally locked with its parent star, so that one side of the planet would remain in perpetual daylight.
But That May Not Be The Case
New observations reveal that in fact 55 Cancri e may have a day and night cycle.
Using both NASA’s Webb Space Telescope observations as well as observations using ESA’s (European Space Agency) Characterizing Exoplanet Satellite (CHEOPS), Stockholm University astronomer Alexis Brandeker and colleagues found that 55 Cancri e has more temperature and orbital variations than they ever expected.
Normally, you would expect a planet orbiting this close to its parent star to be tidally locked with the star so that one side is facing the same way at all times, Brandeker told me during a recent sit-down interview in his office.
But the team has seen temperature variations around the planet which could indicate that it has a normal day-night cycle. That’s a possibility, says Brandeker, because 55 Cancri e may have a slightly eccentric orbit due to long-term gravitational perturbations caused by other members of the planetary system.
The other three known planets of this system orbit much farther out and were discovered years earlier than 55 Cancri e.
It’s All In The Observations
Astronomers can learn a lot about a planet’s surface and putative atmosphere by observing the planet during periods of transitory ingress and egress, as the planet slips behind its parent star and re-emerges from behind its parent star. And that is what the CHEOPS spacecraft has done multiple times since 2020.
We have observed this planet at several different locations and found it is way more variable than we expected, says Brandeker. And frankly more than we can explain at the moment, he says. Our data is still under analysis, so we are not 100% sure that our conclusions are correct, he notes.
How Did 55 Cancri e End Up This Way?
It certainly didn’t form in its present location but likely formed farther out and migrated inward. The details of such migration are not currently known.
One hypothesis is that it was originally a gas giant planet that formed much farther out and then was somehow stripped of it its outer gas envelope, most likely through its parent star’s solar wind. Another hypothesis is that it’s a rocky earth type planet that just happened to form some 8 times more massive than our own planet.
Brandeker favors the latter hypothesis because as he notes, losing a dense atmosphere is not so easy.
I think more likely that it’s just a bare rock and it formed that way, he says.
A generation ago, astronomers were lucky to even indirectly detect any sort of exoplanet in this kind of weird short orbit. Today, extrasolar planetary scientists such as Brandeker routinely take spectroscopy of such close-in extrasolar planets around dozens of stars.
As For 55 Cancri e?
The implication is that you would have a morning where the sun rises and the surface starts to melt and then vaporize to form a silicate mineral atmosphere, says Brandeker. And then in the evening, when the planet cools down again, the vapor atmosphere would rain out as lava, he says.