In our recent look at exoplanets, we emphasized that it's not enough to simply know where a planet is relative to its host star. To understand the planet's properties, you have to know details about its atmosphere among other things. So researchers have done just that, imaging each of the members of a four-planet system called HR 8799, which is centered on a bright young star about 130 light years from Earth.

Their findings suggest that none of the planets' atmospheres look very much like any of the others, and only one of them looks like a member of our own solar system.

Nearly everything about HR 8799 and its surroundings is rather unusual. (The authors refer to all of it as an exosolar system, in parallel to the term exoplanet.) The star itself has been estimated to be as young as 30 million years old. It's only a bit more massive than the Sun, but quite a bit brighter, especially in the UV. It's also much more variable than the Sun, with large changes in its output occurring over the span of just a few days.

But it's the exosolar system aspects that have attracted the most attention. HR 8799 is still surrounded by a debris disk of the sort that is thought to give rise to planets. That disk, however, has been cut into fragments. Below six astronomical units (AU; the typical distance from the Earth to the Sun), the disk is absent, which could imply the presence of one or more as-yet-undetected planets. The disk is present out to about 15 AU, where it goes missing again. In this case, we know why: we've directly imaged four bodies orbiting within the gap in the disk between 15 and 90 AU (one each at 27, 42, and 68 AU, with the fourth's orbit not yet fully characterized).

Note that I didn't use the term planets, because there's an active debate about exactly what these things are, in part because we don't know precisely how old HR 8799 is. If we know how old they are, we can estimate their mass based on how much they've cooled down since their formation. Due to the uncertainties about the system's age, the size estimates have huge ranges. One is between five and seven times the mass of Jupiter, another about 13 to 14 times Jupiter's mass, and a third may be as much as 34 times Jupiter's mass (again, we've not sorted out the fourth object). The higher values place some of these objects squarely in brown dwarf territory—able to fuse deuterium, but not capable of sustaining hydrogen fusion.

If all that weren't confusing enough, attempts to model a system with all these enormous bodies in it have suggested that it's most likely to be unstable. That is, although the objects could have formed at their current distances and may stay there for millions of years, over billions of years, gravitational interactions will force one or more of them to leave their current orbits.

So, HR 8799 was a pretty fascinating system even before a recent set of observations that looked in detail at the individual planets/brown dwarfs. The observations were made using the Palomar Observatory's Hale telescope, which retains the original mirror first commissioned in the 1940s, but has been upgraded with advanced optics. These include a coronagraph to blot out the light of the star, and adaptive optics that sharpen its focus. (In fact, the authors actually used the deformability of one of the mirrors to track the central star their own coronagraph blotted out, by introducing a sine wave deformation into the mirror, which created four evenly spaced images of the star.)

With the hardware and software working in concert, the team captured the spectra of the individual bodies, allowing them to make some inferences about the composition of their atmospheres. And, somewhat surprisingly, they were all different.

Object b contains ammonia and/or acetylene as well as CO 2 but little methane.

but little methane. Object c contains ammonia, perhaps some acetylene but neither CO 2 nor substantial methane.

nor substantial methane. Object d contains acetylene, methane and CO 2 but ammonia is not definitively detected.

but ammonia is not definitively detected. Object e contains methane and acetylene but no ammonia or CO 2 .

Compared to other bodies we know about, only object e looks at all familiar, having a spectrum similar to that seen on the night side of Saturn. As for why they're all so different, the team has no answers, saying, "Whether this is due to small differences in formation, metallicity differences as a function of orbital radius, or evolutionary differences all remain questions."

All of this reinforces the conclusion of the review of exoplanet findings that we linked up at top: until we have a better idea of the diversity of conditions on exoplanets (including their atmospheric composition), it's hard to tell what the conditions on the planet will be like. This study, which is the first to sample the atmospheres of all the known planets in a single system, is an excellent first step towards that. Hopefully, others that follow it will go some way towards helping us understand the diversity of conditions that prevail in exosolar systems.

Astrophysical Journal, 2013. DOI not yet available, but a copy of the paper is hosted here.