Scientists have studied the exoplanet 55 Cancri e, also known as Janssen, to learn how it came to be so close to its host star that its surface is likely an ocean of magma. The researchers analyzed Janssen's orbit and those of other exoplanets around the star and found that Janssen likely formed farther from the star and migrated towards it over time, melting as it moved closer. This study provides insight into how extreme exoplanets like Janssen can form.
The Copernican system, which is home to the "hell planet" Janssen, has some unique quirks among its planetary system. Located about 41 light-years away, the system has six exoplanets in total, including Galileo, Brahe, Harriot, and Lipperhey. All of these planets are more distant from Copernicus than Janssen, which is the closest planet to the star. Astrophysicist Lily Zhao of the Flatiron Institute in New York said that they've learned about how this multi-planet system - one of the systems with the most planets that the team has found - got into its current state.
The Janssen (hell planet) in the Copernican system orbits its star, called Copernicus, every 18 hours. It is 1.85 times the radius of Earth and has a mass of about 8 times that of Earth, making it slightly denser. This means that it could have been a rocky super-Earth at a greater distance from its star. However, the planet's proximity to its star means that temperatures on its surface average at 2,573 Kelvin (2,300 degrees Celsius, or 4,172 degrees Fahrenheit), with the nightside being 950 Kelvin cooler. These temperatures are higher than the melting point of magma, making it likely that the planet's surface is an ocean of molten rock.
Composition of Janssen
It is not clear what Janssen's internal structure is like, but research suggests that it is very different from the rocky planets in our solar system. Because it is difficult to gather information about exoplanets, even ones as close as the Copernicus system, Zhao and her team studied the orbits of the five exoplanets around the star to learn more about Janssen. They found that Janssen's orbit was different from the other four exoplanets, which can be detected based on their effect on the host star. The first way to detect an exoplanet is through transit when the exoplanet passes between us and the star, slightly dimming its light. A regular dip in starlight indicates an orbiting exoplanet.
The second way to detect exoplanets is through radial velocity, which is based on the gravitational effects of the planets on their host star. Each planet orbiting a star has a gravitational pull, which causes the star to wobble slightly. This wobble can be seen in the changes in the wavelength of light from the star, with the light stretching as the star moves away from us (red-shifted) and compressing as the star moves towards us (blue-shifted).
All five exoplanets in the Copernicus system were detected through radial velocity, but further observations revealed that only Janssen and Galileo are seen to transit. This means that it is possible that these two planets do not sit on the same orbital plane as Brahe, Harriot, and Lipperhey. In addition, Galileo's transit is so tangential that astronomers have been unable to measure its radius and temperature, so it is not on the same orbital plane as Janssen. This analysis has been released in the scientific journal Nature Astronomy.
An artist’s impression of the planet Janssen, which orbits its star so closely that its entire surface is a lava ocean that reaches temperatures of around 2,000 degrees Celsius.
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Learning More about the 'Hell Planet'
To learn more about Janssen's orbit, the researchers used a new instrument called the EXtreme PREcision Spectrometer (EXPRES) at Lowell Observatory in Arizona to track the movement of the planet across the star with high precision. This revealed that Janssen traces a path around the star's equator. Previous research had found that the binary companion of Copernicus, a small red dwarf, probably perturbed the system, pulling the exoplanets into an orbital plane that is highly inclined from the star's spin axis.
Zhao and her team believe that an interaction between the exoplanets may have pushed Janssen into a decaying orbit around the star, causing it to fall closer and closer over time. Because Copernicus is spinning, it has a slight bulge around the equator, where the gravitational field is stronger. Janssen was drawn into this region. Galileo may be undergoing the same process on a short 14.7-day orbit, but further analysis will be needed to confirm this. The study shows a way to study the histories of exoplanets with very close orbits around their stars.
The study is particularly interesting for exoplanets called hot Jupiters, which are gas giants with orbits of less than a day. These planets present a puzzle because they are too close to their stars to have a thick atmosphere. Inward migration is one way that these scorching exoplanets could end up so close to their stars. The research suggests that this model may be correct and that the spin-orbit alignment of Janssen supports the idea that gentle migration through tidal dissipation and planetary obliquity tides is a possible explanation for the formation of ultra-short-period planets.
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