55 Cancri e
55 Cancri e (abbreviated 55 Cnc e, formally named Janssen /ˈdʒænsən/) is an exoplanet in the orbit of its Sun-like host star 55 Cancri A. The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice that of the Earth, thus making it the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes fewer than 18 hours to complete an orbit and is the innermost-known planet in its planetary system. 55 Cancri e was discovered on 30 August 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star. In October 2012, it was announced that 55 Cancri e could be a carbon planet.
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 |
Detection method | Radial velocity |
| Orbital characteristics | |
| Apastron | 0.01617 AU (2,419,000 km) |
| Periastron | 0.01464 AU (2,190,000 km) |
Semi-major axis | 0.01544 ± 0.00005 AU (2,309,800 ± 7,500 km) |
| Eccentricity | 0.05 ± 0.03 |
Orbital period (sidereal) | 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 | |
Mean radius | 1.875 ± 0.029 R🜨 |
| Mass | 7.99 +0.32 −0.33 ME |
Mean density | 6.66+0.43 −0.40 g cm−3 |
Surface gravity | 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) |
In February 2016, it was announced that NASA's Hubble Space Telescope had detected hydrogen and helium (and suggestions of hydrogen cyanide), but no water vapor, in the atmosphere of 55 Cancri e, the first time the atmosphere of a super-Earth exoplanet was analyzed successfully.