55 Cancri e

55 Cancri e

Extrasolar planet		List of extrasolar planets
Artist impression of Earth compared to 55 Cancri e with a radius just over twice that of Earth.
Parent star
Star		55 Cancri A
Constellation		Cancer
Right ascension	(α)	08h 52m 35.8s
Declination	(δ)	+28° 19′ 51″
Apparent magnitude	(mV)	5.95
Distance		40.3 ± 0.4 ly (12.3 ± 0.1 pc)
Spectral type		G8V
Mass	(m)	0.95 ± 0.10 M☉
Radius	(r)	1.152 ± 0.035 R☉
Temperature	(T)	5373 ± 9.7 K
Metallicity	[Fe/H]	0.29
Age		7.4–8.7 Gyr
Orbital elements
Semimajor axis	(a)	0.01560 ± 0.00011 AU (2.333 Gm)
		1.27 mas
Periastron	(q)	0.0129 AU (1.94 Gm)
Apastron	(Q)	0.0183 AU (2.73 Gm)
Eccentricity	(e)	0.17 ± 0.04
Orbital period	(P)	0.7365449 (± 0.000005) d
		(17.677 h)
Inclination	(i)	83.4 ± 1.7°
Argument of periastron	(ω)	181 ± 2°
Time of periastron	(T0)	2,449,999.83643 ± 0.0001 JD
Semi-amplitude	(K)	6.2 ± 0.2 m/s
Physical characteristics
Mass	(m)	8.63 ± 0.35 M⊕
Radius	(r)	2.00 ± 0.14 R⊕
Density	(ρ)	5.9+1.5 −1.1 g cm-3
Discovery information
Discovery date		August 30, 2004
Discoverer(s)		McArthur et al.
Detection method		Radial velocity
Discovery site		Texas, USA
Discovery status		Published
Other designations
55 Cancri Ae, Rho1 Cancri e, HD 75732 e
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data

55 Cancri e (abbreviated 55 Cnc e) is an extrasolar planet with half the mass of Neptune orbiting the Sun-like star 55 Cancri A. Its mass is about 7.8 Earth masses (and twice the width of Earth), thus classifying it as the first Super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes less than 18 hours to complete an orbit and is the innermost known planet in its planetary system. 55 Cancri e was discovered on August 30, 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star.

Discovery

Like the majority of known extrasolar planets (found previous to the Kepler mission), 55 Cancri e was discovered by detecting variations in its star's radial velocity. This was achieved by making sensitive measurements of the Doppler shift of the spectrum of 55 Cancri A. At the time of its discovery, three other planets were known orbiting the star. After accounting for these planets, a signal at around 2.8 days remained, which could be explained by a planet of at least 14.2 Earth masses in a very close orbit. The same measurements were used to confirm the existence of the uncertain planet 55 Cancri c.

55 Cancri e was one of the first extrasolar planets with a mass comparable to that of Neptune to be discovered. It was announced at the same time as another "hot Neptune" orbiting the red dwarf star Gliese 436 named Gliese 436 b.

Planet challenged

In 2005 the existence of planet e was questioned by Jack Wisdom in a reanalysis of the data. According to him, instead of the 2.8-day planet there is a planet with a mass similar to that of Neptune in a 261-day orbit around 55 Cancri A. In 2007, Debra Fischer and colleagues at San Francisco State University published a new analysis indicating that both planets existed; the planet in the 260-day orbit was accordingly designated 55 Cancri f.

Transit

The planet's transit of its primary was announced on April 27, 2011. This is one of the few planetary transits to be confirmed around a well-known star, and allowed assumptions about the planet's composition to be made.

Orbit and mass

55 Cancri e is located in a very close orbit around the star which takes less than 18 hours to complete and falls into the category of "hot Super-Earths". A limitation of the radial velocity method used to detect 55 Cancri e is that only a minimum mass can be obtained, in this case around 7.8 times that of Earth, or 48% of the mass of Neptune. Astrometric observations made with the Hubble Space Telescope suggest that the outer planet 55 Cancri d is inclined at 53° with respect to the plane of the sky. If these measurements are confirmed and the system is assumed to be coplanar, the true mass of the planet would therefore be about 25% greater than this lower limit, at around 0.6 Neptune mass. However, a transit has been confirmed, thus its inclination is about 83.4 ± 1.7.

Characteristics

It was initially unknown whether 55 Cancri e was a small gas giant like Neptune or a large rocky terrestrial planet. In 2011, a transit of the planet was confirmed, allowing to calculate its density. The high density calculated suggests that the planet has a "rock-iron composition supplemented by a significant mass of water, gas, or other light elements". A more recent study, based on the Spitzer Space Telescope, suggests that "about a fifth of the planet's mass must be made up of light elements and compounds, including water," according to astronomers.

55 Cancri e receives more radiation than Gliese 436 b.

Formation theories

Prior to the transit observations in April 2011, composition and temperature were unknown. The formation models proposed for Mu Arae c do not apply to a planet this close to its star. Large terrestrial planets may be formed from material compacted by the inward migration of the system's gas giants.

Alternatively 55 Cancri e may be the core of a gas giant pushed inwards before it had time to accumulate a significant gaseous envelope. This was dismissed in 2004, as gas giants can survive for long timescales in the inner regions of a planetary system. However the star could have blown off a volatile layer via coronal mass ejection. In this model (also proposed for Gliese 876 d), the original composition was richer in volatile substances, such as water. As it arrived in range, the planet boiled away to a pressurised ocean of water (in the form of a supercritical fluid) separated from the silicate core by a layer of ice kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containing water vapor and free oxygen produced by the breakdown of water by ultraviolet radiation.

See also

• Mu Arae c

External links

• Jean Schneider (2011). "Notes for Planet 55 Cnc e". Extrasolar Planets Encyclopaedia. http://exoplanet.eu/planet.php?p1=55+Cnc&p2=e. Retrieved 8 October 2011. 
• Extrasolar Visions: 55 Cancri e
• Spitzer Detects a Steaming Super-Earth Eclipsing Its Star (JPL 09.26.11)