55 Cancri e
Artist's impression of 55 Cancri e near its host star | |
| Discovery | |
|---|---|
| Discovered by | McArthur et al. |
| Discovery site | Texas, United States |
| Discovery date | 30 August 2004 |
| Radial velocity | |
| Orbital characteristics | |
| Apastron | 0.01617 AU (2,419,000 km) |
| Periastron | 0.01464 AU (2,190,000 km) |
| 0.01544 ± 0.00005 AU (2,309,800 ± 7,500 km) | |
| Eccentricity | 0.05 ± 0.03 |
| 0.73654625(15) d 17.67711 h | |
| Inclination | 83.59 +0.47 −0.44 |
| 2,449,999.83643 ± 0.0001 | |
| 86.0 +30.7 −33.4 | |
| Semi-amplitude | 6.02 +0.24 −0.23 |
| Star | 55 Cancri A |
| Physical characteristics | |
| 1.875 ± 0.029 R🜨 | |
| Mass | 7.99 +0.32 −0.33 M🜨 |
Mean density | 6.66+0.43 −0.40 g cm−3 |
| 2.273 g | |
| Temperature | 3,771+669 −520 K (3,498 °C; 6,328 °F, dayside) <1,649 K (1,376 °C; 2,509 °F, nightside) |
| Atmosphere | |
| Composition by volume | Likely CO and CO2 |
55 Cancri e (abbreviated 55 Cnc e), formally named Janssen /ˈdʒænsən/, is an exoplanet orbiting a Sun-like host star, 55 Cancri A. The mass of the exoplanet is about eight Earth masses and its diameter is about twice that of the Earth. 55 Cancri e was discovered on 30 August 2004, thus making it the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It is the innermost planet in its planetary system, taking less than 18 hours to complete an orbit. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star.
Due to its proximity to its star, 55 Cancri e is extremely hot, with temperatures on the day side exceeding 3,000 Kelvin. The planet's thermal emission is observed to be variable, possibly as a result of volcanic activity. It has been proposed that 55 Cancri e could be a carbon planet.
The atmosphere of 55 Cancri e has been extensively studied, with varying results. Initial studies suggested an atmosphere rich in hydrogen and helium, but later studies failed to confirm this, instead supporting an atmosphere composed of heavier molecules, possibly only a thin atmosphere of vaporized rock. Most recently as of 2024, JWST observations have ruled out the rock vapor atmosphere scenario and provided evidence for a substantial atmosphere rich in carbon dioxide or carbon monoxide.