55 Cancri

55 Cancri (abbreviated 55 Cnc; Bayer designation ρ¹ Cnc) is a nearby 6th magnitude star in the constellation Cancer. The star is a binary system. The primary component is, like our Sun, a yellow dwarf, but slightly less massive and luminous. Secondary is a distant dim red dwarf. Distance to the system is 41 light years. The brighter component is visible through binoculars or to the naked eye under very dark skies.

Four extrasolar planets have been discovered around 55 Cancri A, making it the largest known extrasolar planetary system as of 2004. Its planets orbit with periods of 3, 15, 44 and 4,520 days. The smallest planet, which orbits the star in just under three days, is about the same size as Neptune, making it one of the smallest extrasolar planets ever discovered. This is the first and so far only known four planet planetary system.

The planets

55 Cancri e

Innermost planet, designated as 55 Cancri e is one of three currently known "hot Neptunes" or "super-Earths" with a mass only 14 Earth masses. Like Neptune, it is a small gas giant – or perhaps more likely – a very large terrestrial planet with dense atmosphere. Distance to the star is less than 1/26th Earth's distance from the Sun; the planet orbits the star in mere 2.8 days. Despite its closeness, the orbit is somewhat eccentric due to the gravitational perturbation caused by the nearby massive planet b.

The planet was discovered by the Hobby-Eberly Telescope in 2004. The telescope is devoted to spectroscopic studies and has one of the most accurate radial velocity instruments.

In 2005, Jack Wisdom questioned the existence of the planet. According to him, the signal caused by the planet actually comes from 55 Cancri c. Instead of the "hot Neptune", he claims that there is a small, 1.8 Neptune mass (31 Earth mass) gas giant in an orbit with a period of about 261 days. [1]

55 Cancri b

Discovered in 1996, 55 Cancri b was the fourth extrasolar planet discovered around a normal star. It is slightly less massive than Jupiter and orbits very near the star, completing one revolution around the star in less than 15 days.

55 Cancri c

55 Cancri c is a small gas giant, having mass about quarter the mass of Jupiter. Having very eccentric orbit, the planet is at apoapsis twice as distant from the star as at periapsis. This would produce extreme seasons on its atmosphere.

Its orbit brings it also very close to planet b, closest distance being only a few million kilometres. However, the planets are very close to 3:1 orbital resonance; if that is the case, they will never be very near each other. Also, the gravity of the star helps keeping the orbits stable.

Despite being third planet from the star, 55 Cancri c is still about one third closer to its star than Mercury is from the Sun.

Of the planets of 55 Cancri, this planet is the least certain. There is a small possibility that the signal interpreted as caused by a planet would be actually from the rotation of the star, estimated to have period between 35 and 42 days.

55 Cancri d

The most distant of known planets orbiting 55 Cancri A, 55 Cancri d orbits at a mean distance of about 5 AU, comparable to Jupiter's distance from the Sun. The orbit is quite eccentric. Despite its mass, about four times Jupiter, it is still one of the best Jupiter analogues.

Astrometric measurements with the Fine Guidance Sensors aboard the Hubble Space Telescope have tentatively detected the planet d. According to the measurements, inclination of the planet – and the whole system – is about 53°. If so, true masses of the planets would be about 25% higher than measured with the radial velocity method – firmly placing the planet candidates into the realm of true planethood.

55 Cancri dust disk

In 1998 discovery a possible dust disk around 55 Cancri A was announced. Calculations gave the disk radius at least 40 AU; also an inclination of 25° was measured. However, the discovery could not be verified and was later deemed spurious.

55 Cancri B

55 Cancri B is a faint 13th magnitude red dwarf. Spectral type of the star is M4 V. It orbits the brighter component at a distance of ~1065 AU, and one orbital revolution lasts tens of thousands of years at least.

See also