55 Cancri e
First super-Earth discovered around a main sequence star.
55 Cancri e, also known by the official name Janssen, is an exoplanet that orbits the Sun-like star 55 Cancri A. It is the closest planet to its star in the system, whipping around it in under 18 hours. This tight orbit makes it one of the hottest exoplanets known, with day-side temperatures exceeding 3,000 Kelvin. The planet’s thermal emission varies, possibly due to volcanic activity, and some proposals suggest it could be a carbon planet.
The planet’s mass is about eight times that of Earth, and its diameter is roughly twice Earth’s. It was discovered in 2004 from data collected between 2003 and 2004, making it the first super-Earth found around a main sequence star, a year before Gliese 876 d. Initially, its orbital period was thought to be about 2.8 days, but observations and recalculations in 2010 revealed the true period of 0.7365 days.
Studies of its atmosphere have yielded shifting results. Early work suggested an atmosphere rich in hydrogen and helium, but later studies failed to confirm that, instead pointing to heavier molecules or possibly just a thin layer of vaporized rock. As of 2024, JWST observations have ruled out the rock-vapor scenario and found evidence for a substantial atmosphere rich in carbon dioxide or carbon monoxide.
The planet’s name, Janssen, was chosen in December 2015 through the IAU’s NameExoWorlds process, following a public nomination and vote. The winning name, submitted by the Royal Netherlands Association for Meteorology and Astronomy, honors Zacharias Janssen, a spectacle maker sometimes credited with inventing the telescope.
The discovery was led by Barbara E. McArthur’s team, using data from the Hobby–Eberly Telescope at McDonald Observatory in Texas. Like most exoplanets found before the Kepler mission, it was detected via radial velocity variations—measuring Doppler shifts in the star’s spectrum. After accounting for three other known planets in the system, a 2.8-day signal remained, indicating a planet of at least 14.2 Earth masses in a very close orbit. The same measurements also confirmed the uncertain planet 55 Cancri c. At the time, 55 Cancri e was one of the first Neptune-mass exoplanets found, announced alongside Gliese 436 b.
In 2005, Jack Wisdom questioned the planet’s existence, suggesting the 2.8-day signal was an alias and proposing a 260-day planet instead. A 2008 analysis by Fischer et al. seemed to confirm both the 2.8-day and 260-day planets, but in 2010 Dawson and Fabrycky showed the 2.8-day signal was indeed an alias, with the true period being 0.7365 days.
A transit of the planet was announced on 27 April 2011, based on two weeks of nearly continuous photometry from the MOST space telescope. The transits matched the period and phase predicted by Dawson and Fabrycky. This made 55 Cancri e one of the few transiting exoplanets around a well-known star, enabling studies of its composition.
The planet orbits at an average distance of 0.01544 AU from its star, taking about 18 hours per orbit. Transit analysis shows its orbital inclination is roughly 83.6°, and it appears nearly aligned with the star’s rotation, with an obliquity of 23° (+14°/−12°). This suggests a dynamically gentle inward migration. It may also be coplanar with the next planet, 55 Cancri b. Given its age and proximity to the star, the planet is almost certainly tidally locked, with one hemisphere permanently facing the star (the dayside) and the other facing away (the nightside).
55 Cancri e receives more radiation than Gliese 436 b. The dayside temperature exceeds 2,000 Kelvin (about 1,700°C or 3,100°F), hot enough to melt iron. Infrared mapping by Spitzer gave an average dayside temperature of 2,700 K and a nightside temperature around 1,380 K. A 2022 reanalysis of Spitzer data found a hotter dayside of 3,770 K and set an upper limit of 1,650 K for the nightside.
Initially, it was unclear whether the planet was a small gas giant like Neptune or a large rocky world. The 2011 transit allowed density calculations, leading to an early suspicion that it might be a water planet. When initial observations found no hydrogen in its Lyman-alpha signature during transit, Ehrenreich speculated that its volatiles might be carbon dioxide instead of water or hydrogen. Another possibility is that 55 Cancri e is a solid planet made of carbon-rich material, unlike the oxygen-rich terrestrial planets of the Solar System.
- mass
- ~8 Earth masses
- diameter
- ~2 Earth diameters
- host_star
- 55 Cancri A
- known_for
- First super-Earth discovered around a main sequence star
Lore & Background
McArthur using the Hobby–Eberly Telescope at McDonald Observatory. The detection was made via radial velocity measurements of its host star. Its transit was confirmed in 2011 by the MOST space telescope, allowing density calculations. The planet's atmosphere has been extensively studied with conflicting results. Early studies suggested hydrogen and helium, but later work failed to confirm this. In July 2014, the IAU launched NameExoWorlds, and in December 2015 the planet was officially named Janssen, after spectacle maker Zacharias Janssen, associated with the telescope's invention. The planet's existence was briefly challenged in 2005 by Jack Wisdom, but later analyses confirmed it.
Reader's Guide
Its discovery marked a milestone in the search for planets between Earth and Neptune in mass, as 55 Cancri e was the first super-Earth found orbiting a main sequence star. The planet’s close orbit and high temperature make it a laboratory for studying extreme planetary environments, including tidal locking, lava oceans, and atmospheric escape. Orbiting its star in less than 18 hours at an average distance of 0.01544 AU, the planet is extremely likely to be tidally locked, with one hemisphere permanently facing the star. Day-side temperatures exceed 3,000 Kelvin, hot enough to melt iron, while infrared mapping has shown significant temperature differences between the day and night sides. The planet’s thermal emission is observed to be variable, possibly as a result of volcanic activity, adding to its scientific interest. Its composition has been debated: initial studies suggested an atmosphere rich in hydrogen and helium, but later work supported heavier molecules or a thin rock vapor atmosphere; most recently, JWST observations have ruled out the rock vapor scenario and provided evidence for a substantial atmosphere rich in carbon dioxide or carbon monoxide. It has also been proposed that 55 Cancri e could be a carbon planet. Its naming after Zacharias Janssen connects it to the history of telescopic observation.
Pioneering the Super-Earth Era
McArthur announced a groundbreaking detection using radial-velocity data collected with the Hobby–Eberly Telescope at McDonald Observatory in Texas. The discovery was announced alongside Gliese 436 b, another close-orbiting hot Neptune. Yet the planet's existence was not settled. The transit was finally confirmed on April 27, 2011, through two weeks of continuous photometric monitoring by the MOST space telescope, validating the corrected period and opening the door to compositional studies.
The Name Janssen
In July 2014, the International Astronomical Union launched NameExoWorlds, an initiative inviting the public to nominate and vote on proper names for select exoplanets and their host stars. Among the candidates was 55 Cancri Ae. In December 2015, the IAU revealed the winning entry: Janssen, a name submitted by the Royal Netherlands Association for Meteorology and Astronomy. The choice honors Zacharias Janssen, a sixteenth-century spectacle maker from the Netherlands who is sometimes credited with an early role in the invention of the telescope. The naming carried a fitting resonance for an exoplanet whose study depends entirely on the light it reflects and emits. By giving the world a human name, the IAU connected a distant, scorching body orbiting a Sun-like star to the long human tradition of looking upward and naming what we find. The name Janssen now appears in official IAU records alongside the designation 55 Cancri Ae, bridging the gap between technical catalog entries and the public's imagination of worlds beyond our own solar system.
A World Forged in Extreme Heat
Its age and closeness make tidal locking virtually certain: one hemisphere permanently bathes in starlight while the other endures perpetual darkness. Those temperatures are sufficient to melt iron. With a mass of roughly eight Earths and a diameter about twice Earth's, the planet sits in a compositional gray zone. Early speculation ranged from a water world to a carbon-rich body in which a third of the mass could be locked in diamond under extreme interior pressure. Ehrenreich noted the absence of hydrogen in the Lyman-alpha transit signature and suggested carbon dioxide might dominate the volatiles instead. The orbital inclination of about 83.6 degrees and a modest obliquity favor gentle inward migration, and the planet may share a plane with its neighbor 55 Cancri Ab.
The Shifting Atmosphere Mystery
For more than a decade, the atmospheric composition of 55 Cancri Ae has defied a single consensus. The earliest analyses pointed toward a hydrogen- and helium-rich envelope, yet subsequent observations failed to confirm that picture and instead favored heavier molecular species, with some researchers proposing nothing more than a thin shroud of vaporized rock. Thermal emission from the planet was noted to be variable, a feature that could hint at volcanic resurfacing on the scorching day-side. Instead, JWST data provided evidence for a substantial atmosphere dominated by carbon dioxide or carbon monoxide, a finding that reshapes models of how such a close-in, high-mass terrestrial world retains its gaseous envelope. The progression—from gaseous giant to water world to carbon planet to a CO/CO2 atmosphere—illustrates how each new instrument refines the picture, leaving 55 Cancri Ae as a living case study in exoplanetary science.
Frequently Asked Questions
How big is 55 Cancri e compared to Earth?
The planet packs roughly eight times Earth's mass into a sphere about twice our planet's diameter. That combination of density and size is what earned it the "super-Earth" label.
How long does it take 55 Cancri e to orbit its star?
A single trip around 55 Cancri A takes fewer than 18 hours—about 0.74 Earth days. Because it sits just 0.015 AU from its host, it is one of the most scorching exoplanets we know of.
Could life exist on 55 Cancri e?
Almost certainly not, given the blistering surface temperature caused by its extreme proximity to its star. Its value to the search for life lies more in showing that rocky super-Earths are common, which helps narrow where we should look for truly habitable candidates.
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