Star of the Month
55 Cancri
Position: 08 hrs 52.3 min +28 deg 20 min.
Due south at 21:18 (GMT) on 15 March 2020
55 Cancri with 53 Cancri to the lower right
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
We now know that many stars—perhaps even most stars—have planets which are known as exoplanets. However the number of such stars which can be easily seen with a small telescope or binoculars (or even with the naked eye in dark areas) are much fewer in number. One such is 55 Cancri which is also known as rho1 Cancri, but this is confusing as rho2 Cancri (aka 58 Cancri) is a fair distance away from rho1 and is not physically connected with it. To find 55 Cancri go to Iota Cancri (a well-known double star) which is at the top of the K-shaped asterism which is the constellation of Cancer and go slightly east (or left) and a bit lower. You will see two fairly bright stars (both roughly sixth magnitude) which are 55 Cancri on the left and 53 Cancri on the right, which are not physically connected either and 53 Cancri is not rho2 Cancri which is below this pair. 55 Cancri is magnitude 6.0 and hence a naked eye star in very dark areas but certainly not in Havering. 55 Cancri is a G8 star which makes it a bit cooler and bit smaller than our Sun which is a G2 star (and rather similar to the secondary star of Algieba; see the double star for March).
55 Cancri has at least five planets. The largest of these planets (55 Cnc-d) is nearly four times larger than Jupiter and orbits at 5.8 AU or just a bit more than Jupiter from our Sun. 55 Cnc-f is much closer to its star at 0.8 AU or somewhat more than Venus with a minimum mass about half that of Saturn or three times that of Neptune. Closer still is 55 Cnc-c with an orbital distance of 0.24 AU or rather closer than Mercury is to our Sun; its minimum mass is rather greater than that of 55-Cnc-f. We now arrive at a hot Jupiter, 55-Cnc-b with a mass of four-fifths of that of Jupiter and an orbital distance of 0.11 AU or just over a quarter of that of Mercury, a tiny 16.5 million kilometres from its star with an orbital period of 14.65 days. But 55-Cnc-e is very close indeed to 55 Cancri, a mere 0.015 AU or 2.3 million kilometres from its star with an orbital period of three-quarters of an earth day. It is much smaller than the other planets with a mass of eight earth-masses or roughly half the mass of Uranus which makes it a super-Earth or a mini-Uranus, but is very dense and may even be as dense as iron and it has been suggested that it could contain diamond. Hubble has detected hydrogen and helium in its atmosphere. Being so close to its star, 55-Cnc-e is almost certainly tidally locked. Its star-facing side is 2573K and its dark side is 1644K—both are extremely hot! On a clear night look up and imagine hot 55-Cn-e whizzing round 55 Cancri.
Artist’s impression of 55-Cancri-e in front of its parent star
Image: ESA/Hubble (via Wikipedia)