Finding solar system analogs is tricky business, as we saw yesterday when examining the discovery of Jupiter and Saturn-class worlds around a distant star. That find, I notice, is getting some attention in the popular media as an indication that our Solar System may not be unique. But take a look at the gas giant recently found around HD 154345 if you want to see an even closer analog to our own system.

HD 154345b is a single world, to be sure, but it orbits a G8 dwarf much more like the Sun than the diminutive star examined yesterday, and it’s a close match for Jupiter not only in size but orbital position. The planet’s minimum mass is 0.95 Jupiter’s, and its 9.2 year circular orbit carries it around its star at a distance of some 4.2 AU. Sound familiar? What’s happening around HD 154345 is more or less what a distant astronomer using our current technologies would see if observing our Solar System.

Rather than using microlensing, the discovery team here put radial velocity techniques to work, with data gathered at the Keck Observatory (Hawaii). Recall that finding ‘hot Jupiters’ is a much simpler proposition, because their tight orbits make the radial velocity signature much easier to spot. In this case, it has taken ten years to tease out the signal of a true Jupiter analog. And as our collected data for other radial velocity projects continues to accumulate, we’re gradually widening our perspective on extrasolar planetary systems to include these farther and perhaps more representative objects.

Thus far in the game, though, we’re looking at only twelve other nearby planets with orbits over six years in duration. And the current find is truly a gem. As the discovery paper notes:

This system joins 55 Cnc in demonstrating that the architecture of the Solar System – a dominant, Jupiter-mass planet at 4-5 AU in a circular orbit with only lower-massed objects interior – while rare, is not unique.

Notice another problematic feature of using radial velocity methods on planets with long orbits. Given the time frames involved, we’re dealing with a single orbit that has consumed a solid nine years, making it difficult to rule out other planets in still more distant orbits that may be detectable over longer time ranges. What it takes to break such a logjam is long-term observation. Transit methods? The large orbital distance makes that possibility unlikely, but we can’t rule out planets further inside HD 154345b’s orbit that could be caught in a transit.

And, of course, by flagging a Jupiter-class planet in a Jupiter-like location, we’ve added another item on our list for future space-borne observation. This G8 star is roughly 59 light years from Earth, bright enough and near enough to make it attractive for such work. Any planetary system with a Jupiter analog in this kind of orbit cries out for investigation of its interior regions, in the hope of discovering the presence of terrestrial worlds in the habitable zone there.

The paper is Wright, Marcy et al., “The Jupiter Twin HD 154345b,” accepted at Astrophysical Journal Letters and available online.