Confirmed exoplanet · Database record + computed analysis
55 Cnc e
A confirmed world orbiting 55 Cnc, 41.0 light-years away.
A super-Earth
1.88 Earth radii. Worlds between Earth and Neptune in size are the most common in the galaxy, yet our Solar System has none, which is why each one matters. With 8.0 Earth masses packed into that size, its density points to rock and iron rather than gas.
The orbit
Its year lasts about 17.7 hours, making it an ultra-short-period planet skimming its star.
Temperature and starlight
Its equilibrium temperature (the airless-world estimate) is about 1,685 °C. Hot enough to melt copper; any surface would glow. It receives 2,657.8 times the starlight Earth gets.
Its sun
Its star is an orange K-type (5,172 K), G8 V: cooler and longer-lived than the Sun, a class some astronomers consider ideal for habitability. The system holds 2 stars, so its skies have more than one sun. It shares the system with 6 other known planets.
How far is that, really?
At 41 light-years, tonight's light from this system left it around the year 1984. That is close by galactic standards, which makes it a prime target for follow-up telescopes. A Voyager-speed probe would need roughly 726,253 years to get there.
How we found it
It was found by the radial-velocity method: the planet's gravity swings its star in a small circle, and that wobble shows up as a rhythmic Doppler shift in the starlight. The wobble's size gives the planet's minimum mass. The discovery was announced in 2004 from McDonald Observatory.
Everything above is computed from the archive’s measured values for this planet. Where a quantity is missing, the statement is omitted rather than guessed.
The deep dive
Researched for the Atlas from Wikipedia — 55 Cancri Ae (9,623 characters read) · updated Sep 20, 2026
01 First Super-Earth Around a Sun-Like Star
When Barbara E. McArthur and her team announced 55 Cancri e in 2004, it earned a milestone that still stands: the first super-Earth discovered orbiting a main-sequence star. The team used the Hobby–Eberly Telescope at McDonald Observatory in the Davis Mountains of Texas, measuring the Doppler shift of starlight to detect tiny wobbles in 55 Cancri A caused by the planet's gravitational pull. At announcement, three other planets were already known in that system, and only after their signals were subtracted did a stubborn leftover oscillation at around 2.8 days emerge — pointing to a planet of at least 14.2 Earth masses in a very tight orbit. The discovery was announced simultaneously with Gliese 436 b, another close-orbiting world dubbed a "hot Neptune," which gives a sense of how rapidly this class of planet was being found in that era. Beating Gliese 876 d to the title by a full year, 55 Cancri e opened the observational era of super-Earths around stars like our own Sun.
02 How a Ghost Period Fooled Everyone for Six Years Deeper
The planet's discovery came packaged with a significant error that persisted for six years. The initial orbital period derived from radial-velocity data was about 2.8 days — a number that appeared in the literature and shaped early estimates of the planet's mass and temperature. In 2005, Jack Wisdom questioned whether this signal was real or an alias, a mathematical artifact produced when an underlying periodic signal beats against the cadence of the observations themselves. Wisdom also separately proposed a 260-day planet in the system. A 2008 reanalysis by Fischer and colleagues seemed to vindicate the 2.8-day period, but in 2010 Dawson and Fabrycky demonstrated conclusively that it was indeed an alias. The true orbital period turned out to be just 0.7365 days — roughly 17.7 hours — almost exactly one-quarter of the spurious value. This correction dramatically changed the planet's implied proximity to its star and, consequently, every physical quantity tied to orbital distance. The episode is now a textbook example of alias contamination in radial-velocity planet searches, underscoring how the spacing of observations can silently manufacture false periodicities.
03 An Orbit Completed Before Breakfast, Lunch, and Dinner
With an average orbital distance of just 0.01544 ± 0.00005 AU — less than one-sixtieth the distance between Earth and the Sun — 55 Cancri e laps its star in under 18 hours. To put that in everyday terms, the planet completes roughly one and a third full orbits during a single human workday. The transit geometry reveals an orbital inclination of about 83.6°, nearly edge-on as seen from Earth, and the planet's orbital plane appears close to alignment with the rotation axis of its host star, with an obliquity of 23 degrees (with an uncertainty of +14° and −12°). This near-alignment favors models in which the planet migrated inward gradually through dynamically gentle processes rather than through violent gravitational scattering. The planet may also be coplanar with the next planet outward in the system, 55 Cancri b, suggesting a shared and relatively orderly migration history. Transit observations confirming this geometry were announced on 27 April 2011, based on nearly two continuous weeks of photometric monitoring with the MOST space telescope.
04 Tidal Locking and a Permanent Day Side
Given 55 Cancri e's extreme closeness to its star and the system's considerable age, scientists consider it essentially certain that the planet is tidally locked. This means one hemisphere — called the dayside — permanently faces the star, baking under continuous irradiation, while the opposite nightside is in permanent darkness. The temperature contrast between these two hemispheres is staggering. Infrared mapping with the Spitzer Space Telescope originally measured an average dayside temperature of 2,700 K (2,430 °C; 4,400 °F) and a nightside temperature of roughly 1,380 K (1,110 °C; 2,020 °F). A 2022 reanalysis of that same Spitzer data pushed the dayside figure even higher, to 3,770 K (3,500 °C; 6,330 °F), while placing an upper limit of 1,650 K (1,380 °C; 2,510 °F) on the nightside. At those dayside temperatures — exceeding 3,000 K in some measurements — common rock-forming minerals and even iron melt or vaporize, creating conditions with no Solar System parallel.
05 Diamond Mantle or Carbon Planet? Deeper
One of the most dramatic hypotheses about 55 Cancri e's interior is that it may be a carbon planet — a world whose bulk composition is dominated by carbon-rich minerals rather than the silicate and oxide minerals that make up Earth and the other rocky planets of our Solar System. In this model, roughly a third of the planet's mass would be carbon. Because of the enormous pressures and temperatures in the planet's interior, a substantial portion of that carbon could exist in the form of diamond. The planet has a mass of about eight Earth masses and a diameter approximately twice Earth's, which constrains its bulk density and makes a purely gaseous or water-dominated composition less likely, though not impossible. Initial transit observations ruled out a thick hydrogen envelope but at first left open the possibility of a water-world or an exotic carbon-rich interior. Further observations remain necessary to confirm which scenario is correct. The carbon-planet hypothesis depends critically on the carbon-to-oxygen ratio in the protoplanetary disk from which 55 Cancri e formed — a ratio that is still debated for this particular stellar system.
06 A Shifting Atmosphere: From Hydrogen to CO2 Deeper
Few exoplanet atmospheres have generated as many contradictory headlines as 55 Cancri e's. In February 2016, NASA announced that the Hubble Space Telescope had detected hydrogen cyanide but no water vapor, implying an atmosphere rich in hydrogen or helium — marking the first successful atmospheric analysis of a super-Earth. Yet a spectroscopic study from 2012 had already failed to detect escaping hydrogen, and a 2020 study found no escaping helium, collectively suggesting there is no primordial hydrogen-helium envelope. In November 2017, Spitzer infrared data pointed toward a global lava ocean cloaked by an atmosphere at roughly 1.4 bar of pressure, potentially containing nitrogen and possibly oxygen. Then, in May 2024, JWST observations using its Near-InfraRed Camera and Mid-Infrared Instrument produced a thermal emission spectrum spanning 4 to 12 micrometers. This new dataset ruled out a thin atmosphere made of vaporized rock and instead provided evidence for a substantial volatile atmosphere likely rich in carbon dioxide or carbon monoxide — possibly sustained by outgassing from a subsurface magma ocean.
07 Volcanic Activity Written in Variable Heat
The planet's thermal emission, measured in infrared light, does not stay constant — it fluctuates in ways that have captured scientists' attention as possible evidence for large-scale volcanic activity. The interpretation is that massive eruptions could release enormous clouds of dust that periodically shroud the planet, blocking the thermal radiation that telescopes detect. By 2022, researchers had also noted a large variability in the depth of the planet's transits across its star's disk. Transit depth can change if the planet's apparent size or opacity changes, and volcanism on a planetary scale could plausibly do exactly that. An alternative explanation is equally exotic: the presence of a variable gas torus sharing the planet's orbit — a ring of material co-orbiting with 55 Cancri e that periodically thickens or thins. Distinguishing between these two mechanisms requires continued monitoring, and neither explanation has been definitively confirmed. The variability itself, however, is well established in the data.
08 Naming Janssen: Honoring a Telescope Pioneer
The planet carries the official IAU-approved name Janssen, assigned through the NameExoWorlds initiative that the International Astronomical Union launched in July 2014. The process invited public nominations and a global vote. In December 2015 the IAU announced the winner: Janssen, a name submitted by the Royal Netherlands Association for Meteorology and Astronomy on behalf of the Netherlands. The name honors Zacharias Janssen, the spectacle maker from the Netherlands who is sometimes credited with inventing the telescope — an instrument whose descendants, four centuries later, would be used to characterize the very planet bearing his name. The naming is especially fitting given that the 2024 JWST observations, made with one of the most powerful optical systems ever built, represent the most detailed look yet at Janssen's atmosphere. The host star 55 Cancri A was given a separate name through the same process.
09 Searching for Magnetic Star-Planet Interactions Deeper
Because 55 Cancri e orbits at less than 0.1 AU from its host star, researchers have considered whether the planet's magnetic field — if it has one — might interact with the star's own magnetic environment strongly enough to trigger observable effects. One predicted signature of such magnetic star-planet interaction is stellar flaring that repeats in lockstep with the planet's orbital period, potentially producing coronal radio emissions. A dedicated search for exactly these signals was carried out in 2011. Sensitive radio observations were made to look for periodic bursts synchronized to the planet's orbital rhythm of roughly 0.74 days. The search returned no detected signal. This null result does not rule out such interactions entirely — the emission could be below detection thresholds, or the geometry may be unfavorable — but it sets an upper bound on the strength of any radio emission driven by the planet-star magnetic coupling. Magnetic interactions between close-in planets and their host stars remain an active area of theoretical and observational research across the broader exoplanet community.
10 How the MOST Telescope Confirmed the Transit
Confirming that a planet actually passes in front of its star — a transit — is a separate and demanding step from detecting the planet through Doppler measurements. For 55 Cancri e, this confirmation came on 27 April 2011, when researchers announced a successful detection using the MOST space telescope. The key was nearly two continuous weeks of photometric monitoring: staring at 55 Cancri A and recording tiny dips in brightness with high precision. The transits arrived with the period of 0.74 days and at precisely the orbital phase that Dawson and Fabrycky had predicted from their 2010 radial-velocity reanalysis — an independent confirmation that their corrected period was right. Transit detection is valuable far beyond simply verifying existence. Because a transit measures the physical size of the planet as it blocks a fraction of the star's disk, combining that size with the mass from radial velocities gives the planet's density — and density is the first clue to whether a world is rocky, watery, or gaseous. The 55 Cancri e transit is described as one of the few confirmed around a well-known bright star.
11 A Possible Hydrogen Refuge on the Nightside Deeper
One creative model proposed to reconcile conflicting atmospheric measurements invokes the planet's tidal locking in an unexpected way. Because 55 Cancri e permanently faces its star on one side, the tidal, orbital, and rotational centrifugal forces combine asymmetrically across the planet. Theorists have proposed that this force environment could partially confine a hydrogen-rich atmosphere on the cooler nightside. In such a scenario, heavier volcanic molecules would be held within latitudes less than 80° from the anti-stellar point, while the lighter, more volatile hydrogen would not be as constrained. Hydrogen would slowly diffuse toward the dayside, where intense X-ray and ultraviolet irradiation from the star would destroy it. The model predicts that hydrogen might be detectable in small quantities — consistent with observations that claimed to find it — while also explaining why other studies measured no significant rate of hydrogen escape. For this mechanism to operate, the planet must have become tidally locked before it lost its entire original hydrogen envelope, a condition that depends sensitively on the system's age and the planet's early orbital evolution.
12 Open Questions After Two Decades of Study
Despite 55 Cancri e being one of the most studied exoplanets in the sky, fundamental questions remain stubbornly open. The planet's bulk composition — whether it is a carbon-rich world with a possible diamond interior, a silicate rock, or something more exotic — has not been settled by observation. Its atmosphere has been described in contradictory terms by different instruments and techniques over more than a decade, and while the 2024 JWST results represent the strongest evidence yet for a substantial volatile atmosphere rich in carbon dioxide or carbon monoxide, the precise composition is still being worked out. The source of the planet's variable thermal emission — whether giant volcanic episodes or a co-orbital gas torus — remains unresolved. It is also unclear whether the planet retains any primordial hydrogen or helium, given that two independent studies failed to detect escaping gas of either type. With JWST now able to produce thermal emission spectra in detail that was unimaginable a decade ago, 55 Cancri e is positioned to answer at least some of these questions in the coming years.
About this record
The measured values come from the NASA Exoplanet Archive, and any missing value has not been determined yet; the Atlas never fills gaps with guesses. More standout worlds: Proxima b, TRAPPIST-1e, K2-18 b, 55 Cancri e, and the detection methods that found them all.