Let’s face it: Most planets are death traps. Of the thousands of worlds known to science, only a handful are considered potentially habitable to life. The remainder would kill any visitors in a variety of nightmarish ways. And yet even with this tough competition, the planet 55 Cancri e, also known as Janssen, has distinguished itself as one of the galaxy’s ultimate “hell planets.”
This searing “Super-Earth,” located 41 light years from Earth, is so close to its star that its year lasts only 18 hours. Scientists surmise that its surface is probably engulfed in a lava ocean that is 2,400°F on its night side—and twice as hot on the day side. But though 55 Cancri e is an iconic and well-studied world, the story of how it came to be such an infernal wasteland has remained shrouded in mystery.
Now, scientists led by Lily Zhao, a postdoctoral research fellow at the Flatiron Institute's Center for Computational Astrophysics, have achieved a breakthrough in our understanding of 55 Cancri e, and other ultra-short-period planets (USPs) that have years shorter than a day.
These USPs were not always so “hot hot hot!” as Zhao described 55 Cancri e in an email to Motherboard, because they probably formed farther from their stars, before migrating into the hot seat. Understanding these movements can help scientists probe the unimaginably diabolical conditions on USPs, but they could also shed light into the origins of habitable planets, which is a major step in assessing the odds that alien life has emerged elsewhere in the universe.
“Understanding how planets evolve and migrate will definitely impact our understanding of planetary habitability likelihoods,” Zhao told Motherboard. “The habitable zone is a good rule of thumb for whether a planet could sustain water-based life at present, but in order for life to successfully form, a planet must stay habitable for the time it takes life to manifest. It is therefore essential to understand how planets might move around in systems with different numbers and types of planets as well as around different stars to understand how long a planet has been and/or will stay habitable.”
With this in mind, Zhao and her colleagues used a sophisticated new instrument called EXtreme PREcision Spectrometer (EXPRES) at the Lowell Observatory’s Lowell Discovery Telescope in Arizona to snag an unprecedented measurement that places “constraints on theories for how USPs migrate to their present day positions and how they interact with other planets in compact multiplanet systems,” according to a study published on Thursday in Nature Astronomy.
“55 Cancri e is so interesting both because it orbits so close-in to its host star and because it's one of the smaller planets to do so,” Zhao said. “Its radius is less than twice the radius of Earth, meaning when it transits (i.e., passes between its host star and Earth) it blocks a very small portion of light from the host star. The nature (more specifically the Doppler shift) of this missing light is what needs to be captured in order to get this measurement, and it has to be done well for the entirety of the transit.”
“The observations captured by previous instruments simply weren't capable of detecting a signal this small,” she continued. “EXPRES was specifically designed to return more precise measurements capable of capturing signals of this level. This measurement is a great benchmark for the capabilities of EXPRES, demonstrating that we are now capable of detecting signals of the magnitude needed to find Earths elsewhere, planets that would be hospitable to life.”
55 Cancri e has fascinated and puzzled scientists ever since it was first discovered in 2004. Scientists have speculated that it could be a so-called “carbon planet,” meaning a weird type of world with more carbon than oxygen that might contain enormous volumes of diamonds, like some kind of dragon lair in space.
“Diamond is one possible explanation for the size and mass of 55 Cancri e, but we really can't be sure what this planet is made of,” Zhao said. “In fact, the composition of smaller, low-mass planets like 55 Cancri e is not currently well understood because it has been difficult to measure the mass of these planets.”
“With the potential for more precise [radial velocity] measurements using EXPRES, we will be able to discover and determine masses for more of these planets and get a better grasp of what these planets might be made of,” she added.
This famous hell world is also perplexing because of its peculiar alignment relative to the plane of its star system. Scientists know that other planets orbit the same star due to their subtle gravitational signatures, but 55 Cancri e is the only one that passes in front of the star, from our perspective on Earth, suggesting that it occupies a unique angle compared to its planetary siblings.
Previous research has pinpointed a few different explanations for this strange alignment, which has also been observed in other systems with USPs. One hypothesis suggests that USPs are worlds that formed farther out in their star system, but were nudged closer to their star in a chaotic process driven by gravitational interactions with other planets that have tilted “eccentric” orbits.
A second model also envisions the planet originating at a larger distance from the star, but invokes a gentler migration driven by “obliquity tides” that arise from the angle between the planet’s spin axis and the planet’s orbital plane. This model, which does not require highly eccentric planetary orbits, predicts that a migrating planet might become synced with its star, while ending up misaligned with the orbits of other planets in the system.
The second “low eccentricity” model matches the observations that Zhao and her colleagues captured of 55 Cancri e using EXPRES, a finding that sheds new light on the origin of this searing planet.
“This theory posits that the planetary system is initially misaligned with the host star,” Zhao explained, adding that these skew dynamics may be caused by the presence of a second star in the 55 Cancri system. “The inner planet will likely start out aligned with the other, outer planets, but as the planet moves closer in, it will become more and more aligned with the star.”
“We find that 55 Cancri e's orbit is well aligned with its host star's equator, which is expected out of the low-eccentricity and obliquity-tides models,” she continued. “This result therefore favors those two theories…over the high-eccentricity pathway. That's not to say the high-eccentricity pathway may not hold true for other systems! But for 55 Cancri e, our results suggest it was a more dynamically gentle process that brought the planet so close into its host star.”
The new study demonstrates that EXPRES can detect tiny signatures from USPs that were previously out of observational reach, revealing new insights about these tortured worlds. Zhao and her colleagues plan to continue using next-generation instruments to extract the hidden secrets of planetary systems across the Milky Way. These efforts may reveal some of the conditions required to make “heaven” planets that could potentially support life, but in the meantime, Zhao will always have a soft spot for the aggressively inhospitable world of 55 Cancri.
“Subjectively speaking, it is definitely my favorite hell planet!” she said. “I think what makes it perhaps particularly hellish is all the other planets in the system, some of which even flirt with the habitable zone. I feel like being covered in lava looks even worse next to a host of more conventionally terrestrial planets!”