Planetary Mysteries

Are we getting blasé about the discovery of planets outside of our solar system? I hope not, because for every dozen or so-ho-hum planetary systems that astronomers find in our galactic neighbourhood, there seems be at least one that makes them scratch their heads and ask, “How do you get a solar system like that?”

Let’s back up just a moment. It wasn’t that long ago that the only planets we knew about were the nine – or make that eight, with the demotion of Pluto – within our solar system. But over the last decade or so, astronomers have used a variety of techniques to detect planets orbiting stars beyond our solar system, and by now more than 450 have been found. (I reported on some of those methods in a feature story in the Winter 2006 issue of the magazine.)

That growing tally of planets includes a giant world orbiting a sun-like star with the not-so-catchy name of 1RXS J160929.1-210524 (or 1RXS J1609for short). This particular planet has the distinction of being the first to have been directly imaged, thanks to U of T astronomers Ray Jayawardhana and Marten van Kerwijk, along with David Lafrenière of the Université de Montréal. The team used the Gemini North telescope (equipped with adaptive optics) on Hawaii to snap a photo of the star and its chubby companion back in 2008.

At that time, however, they could only guess at the planet’s mass, and, in fact, they couldn’t be sure that the star and its companion really were part of a single system. Now, after studying the “proper motion” of the objects – their apparent motion across the sky – they say it really is a star-and-planet system, bound by mutual gravitation. And they’ve pinned down the mass of the planet: It’s about eight times bigger than Jupiter.

What’s so unusual about that? Well, the planet is orbiting its host star at a distance some 300 times greater than the distance between the Earth and our sun. Although the planet is much larger than our own, it’s actually the smallest planet known to be circling its host star at such a great distance – and this is something of a dilemma for our understanding of how planetary systems form and evolve.

“It’s the nature of the beast that’s the big surprise,” Dr. Jayawardhana said in an e-mail, calling it a “challenge to planet formation theories.”

The favoured theory of planetary formation is a process known as “core accretion,” in which a planet starts off as a small, rocky core, that grows gradually as it draws in gas and dust from its surroundings. But that method seems unlikely to result in such a small planet in such a wide orbit, Dr. Jayawardhana says. One possibility is that it formed closer to its host star, and was then hurled outward as a result of gravitational interaction with other planets in the system – however, Dr. Jayawardhana and his colleagues haven’t yet found any other sizable planets in the 1RXS J1609 system.

There are other, competing theories as well. The system could have formed by means of a contracting cloud of gas, the way binary star systems are believed to form. Or it could have resulted from the fragmentation of a so-called “protostellar disk.” The bottom line is that astronomers don’t yet have a definitive explanation for the formation of planetary systems – one that can account for the stunning diversity seen among these new extrasolar worlds. Which means that the list of puzzles is growing almost as fast as the tally of extrasolar planets.

Dr. Jayawardhana and his colleagues describe their latest findings in an article submitted to the Astrophysical Journal.