There are some astronomers who believe our Solar System is a rare example, if not unique. For all the extrasolar planets which have been discovered up to the time of writing (late 2009), none of them match the Earth, and astronomers are still seeking a solar system which mirrors or closely resembles ours. Of course, they may never find one. It is a humbling thought that the Earth holds the only life in the entire universe, so far as we know, which makes our home planet all the more precious. However, as technology advances to the point where terrestrial (rocky) planets can be detected, the possibility of finding another oceanic world similar to Earth remains open.

Planet Hunters

The BBC's Horizon programme broadcast an episode entitled 'Planet Hunters' in 2000. One of the participants, Professor Geoff Marcy of the University of California, Berkeley, said:

'Since I've been a little child I thought to myself wouldn't it be wonderful if we could learn whether or not there are other planets out there like our own and I thought if so, all of the science-fiction novels that we all read might, in fact, have some bearing on reality. There may, in fact, be other beings out there who are indeed thinking about us and wondering if we're here.'

Another contributor, Dr Andrew Collier Cameron of the University of St Andrews, said:

'Once every few days I get the chance to sit back for five minutes and I think how incredibly lucky we are to be living in a time when we've been able to make this first step and when we know that the Holy Grail is perhaps only 10 or 12 years away and it's going to happen in our lifetimes. We're going to be able to detect Earth-like planets within the next 10 or 15 years and we're going to be here and we're going to be part of it. That's pretty exciting. There can't be many more exhilarating things to be doing with your life.'

Almost a decade since that programme was broadcast, terrestrial planets are indeed being detected by various means including astrometry, radial velocity, microlensing and transit methods - all very painstaking and laborious, but very rewarding work. The mu Arae planetary system discussed below is just one system with four planets detected between 2000 and 2006. Professor Marcy's team discovered two of these planets.

Mu Arae

Mu Arae (μ Arae) is a +5.15 magnitude, yellow dwarf star in the Southern Hemisphere constellation   Ara 'the Altar'. The star's co-ordinates are: RA 17^h 44^m; Dec −51°50'. The star, also known as HD 160691 on the Henry Draper¹ catalogue, is like the Sun and only 49 light years² distant from Earth. Simply put, if you could board a rocket which is capable of travelling at the speed of light, the round trip to mu Arae and back would take about a century. Astronomers though, used as they are to studying galaxies millions of light years distant, regard mu Arae as a close neighbour.

Surrounding the star is a family of planets: the most similar to our own Solar System of anything found thus far. The discoveries catapulted mu Arae into the top 100 target stars for NASA's Terrestrial Planet Finder (TPF), although the astronomical funding required for this project remains a controversial subject and no date for launch has been fixed at the time of writing.

The Planets

The nomenclature that has been decided upon for planets outside of our solar system is to use a lower-case Roman letter after the parent star catalogue number (or name), for example 'mu Arae b'. This name stays with the planet even if further planets are discovered in the same system, and despite the position of the new planet relative to the star. Therefore the first-discovered planet of mu Arae is mu Arae b even though mu Arae c has a closer orbit to the shared star mu Arae.

The habitability zone of mu Arae ranges from 0.7 AU to 2 AU (1 AU is the mean distance from the Earth to the Sun, 93m miles or 150m km), and two planets, b and d, share this fabled 'Goldilocks Zone'. The order of the planets in terms of distance from the star (as at 2006) is: planet c, planet d, planet b, and planet e. If other planets are detected in the future, they will be catalogued planet f, g, h, etc. Up to 2009 we only know of one planet labelled f, which belongs to the 55 Cancri A system, and there are, at the time of writing, no planets labelled g.

Mu Arae b

Planet mu Arae b was discovered in 2000 by American planet hunters Dr Paul Butler and Professor Geoffrey Marcy, who detected it by examination of the star's radial velocity. The pair were awarded the Henry Draper Medal in 2001. At the time of its discovery, astronomers guessed there were other planets in the system:

Not only did the Harps measurements confirm what we previously believed to know about this star, but they also showed that an additional planet on a short orbit was present.
– Francois Bouchy of the Marseille Astrophysics Laboratory

Planet b is within the outer edge of the system's 'Goldilocks Zone' (habitable zone), but as a gas giant it's not a candidate for the search for life (as we know it).

Mu Arae c

Planet c's discovery was made by a team working at the European Southern Observatory at La Silla in Chile in 2004. At that time it was a record-breaker - the smallest planet ever detected in orbit around a main-sequence star. Astronomers are excited about this particular planet because it is a rocky world like the Earth, although it is much bigger, over ten times greater in size. These types of planets are called a 'Super Earth'. That doesn't mean that they resemble our lush, life-abundant world though. Unfortunately, mu Arae c orbits far too close to its star for water to be found in its liquid form. Its surface temperature has been estimated at a fervent 1,160°F (900 Kelvin), thereby crossing it off the list of possibilities for alien life.

Mu Arae d

The planet mu Arae d is just inside the system's habitable zone, but as a gas giant it's not a candidate for the search for extra-terrestrial life. However, should the planet have a rocky moon with enough mass to retain an atmosphere, possess suitable gravity, and have a magnetic field to shield it from the parent star's radiation, that would make it a distinct possibility.

Mu Arae e

Planet mu Arae e orbits at almost exactly the same distance from its star as Jupiter does from our own Sun - they each take approximately 11 Earth years to complete one year. It is classed 'superjovian' as its mass is almost twice that of Jupiter. Planet e was discovered by Butler and Marcy, who also detected planet b.

The planets were all given proper names by the IAU in 2015. They honour literary characters from Don Quixote.

Other Discoveries

The four-planet system of mu Arae is not a record-breaker; the current leader, up to 2009, is 55 Cancri A which boasts five planets. This system also has a superjovian planet in an extreme orbit, which astrophysicists think is beneficial to inner planets. Large gas giants act like vacuum cleaners, hoovering up hazardous debris which might be detrimental to smaller worlds. The fifth planet discovery in the 55 Cancri A system happens to orbit in the habitable zone, although it is gaseous, not a terrestrial world.

Professional astronomers are hopeful of detecting more planets in the mu Arae system, and millions of amateur astronomers wait with bated breath for the announcement of a right-size rocky planet in a circular orbit that's not too close and not too far from its star.