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Jon Teamlaverne Jon Teamlaverne | 14:17 UK time, Friday, 10 September 2010

After four weeks of sending messages in the direction of Upsilon Andromedae, we're moving on to a new star. Friend of the show and presenter of The Sky At Night Paul G.Abel has kindly written us this piece about the new destination.


47 Ursae Majoris: An Abode for Life?


The star 47 Ursae Majoris is a sun-like star which lies some 46 light years away in the constellation of Ursa Major. This constellation also goes under several names: "The Plough" and "The Great Bear", and the French call it 'La Casserole" and to be fair it does resemble a large saucepan far more than an old fashioned Plough!


The star is rather difficult to see with the naked eye as it is listed as apparent magnitude +5 (apparent magnitude being a star's brightness as seen from Earth), so for this reason we haven't included a star chart. The star is classed as a dwarf G class star which means it is of similar size to our own sun and is a relatively cool yellowish colour. Physically, we know temperature of a star is related to its colour. This can be demonstrated quite easily. For example if you were to get a iron poker and put it in a fire, the poker would start to glow a dull red, as the fire heats the poker further it begins to get a bright red, until it glows yellow, then white, then finally whitish blue. So, the dull red colour is a lower temperature than the brighter red, the yellow is cooler than white and so on, and so it is with stars. The surface of our sun is warmer than the surface of the older orange star Arcturus, but Vega a bluish white star is much hotter than our own sun.


When we are looking for planets which can support Earth type life (by this, I mean life as human beings can understand it), it is important that the parent star is stable and old enough for planets to have formed and be in stable orbits. 47 Ursae Majoris was one of those early stars identified by Astronomers Geoff Marcy and Paul Butler as having a star probably having a planet in orbit- thus came the discovery of 47 Ursae Majoris-b.


47 Ursae Majoris-b


Estimates place 47 UMa-b to be about 2.5 times the size of Jupiter- pretty big, and almost certainly a gas giant like Jupiter and Saturn in our own Solar system. It's estimated to be about 2.1 AU (Astronomical Units) from the parent star - an AU being the average distance between the Earth and Sun. The planet takes some 2.95 years to orbit 47 Ursae Majoris and if it were located in our own solar system then it would be found between the orbits of Mars and Jupiter.


Can Earth-life exist here? Well, 47UMa b is a gas giant with no solid surface, however the gas giants of our own solar system have an enormous collection of satellites, so there may well be some large satellites in attendance. The planet itself does lie on the outer edge of the star's habitable zone (the zone around a star in which liquid water can exist), however it has been hypothesized that the reflected light and heat from 47 UMa-b along with the effect of gravitational-tidal heating may heat the nearest moons sufficiently so that liquid water could exist there. So there could be moons around this gas giant with earth like conditions. Can you imagine living on such a world? The sky would be spectacular, the enormous 47 Ursae Majoris-b in the sky, perhaps with a magnificent ring system casting dramatic light and shadows on the surface!

The following artwork of 47 Ursae Majoris-b, by James Westerman, shows some hypothetical terrestrial planets orbiting close to the star itself.


Other planets in the System?


It is thought that two other planets are to be found within the system The second planet 47 UMa-c lies further out than b, but there is some contention as to just how far out it is, while a third planet UMa-d (discovered by D. Fischer and P Gregory) lies at an even greater distance - 11.6 AU and takes just over 38 years to orbit the parent sun. Both of these planets are thought to be gas giants but lie well away from the habitable zone and so they must be cold dark places indeed!


We cannot rule out the possibility of terrestrial planets (i.e. planets like Mercury, Venus, Earth and Mars) existing closer in than b within the stars habitable zone where liquid water could exist and so this planet is on the target list for NASA's terrestrial planet finder mission which hopes to find planets like the earth orbiting other stars.


So, if there's anyone like you and me in this system, they won't receive our 'Wow-signals' for 46 years after transmission because radio waves travel at the speed of light and 47 UMa is 46 light years away. If our Ursae Majoran is prompt and replies straight away, perhaps with some Ursae Majoran jazz or a recipe for Ursae Majoran Gin and Tonic, then it will take another 46 years for that transmission to reach Earth, this means it takes a total of 92 years for a quick chat! Best to get them to leave a message...



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