Some solar systems have a lot of planets packed in close to their stars. Others are just the opposite. HR 8799 is a young star that is more massive than our Sun. It’s so young–about 30 million years–that it’s planetary system is still forming, and it will probably only live for 2 or 3 billion years.
But HR 8799’s young planetary system is filled with surprises. That’s not an illustration up there. That’s an actual infrared photograph takes by the Keck Telescopes. It’s possible to take this picture because the star’s four planets are young and hot, so they show up in infrared, and because they orbit so far from the star. The innermost planet orbits farther than Saturn does from our Sun, and the outermost planet is more than twice as far as Neptune.
That’s a lot different from our own Solar System. In fact, we think that Neptune started quite a bit closer, then moved out later. It’s not clear how these four planets got where they are. And they’re big planets, too. All four are 5-10 times the mass of Jupiter. Our own Solar System didn’t even have that kind of mass available.
So what does this mean? Well, we’re not seeing the finished product here, so it’s hard to tell what we’re seeing in terms of the other, older planets we’ve discovered. Perhaps the massive outer debris disk around the star will scatter the planets into deep space, and it will start to look more like the other solar systems we’ve seen. Or perhaps the planets will stay close where they are, like Fomalhaut b. Fomalhaut b has an even wider orbit, nearly four times the size of Neptune’s, and it’s elliptical, so it does appear to have been scattered some.
Or maybe some stars have different kinds of debris disks than others and produce different kinds of planetary systems. It’s too early to tell. But if we can better understand what’s going on with HR 8799, we’ll have made a big step toward figuring out how all planets form.