In late 2008, astronomers announced the discovery of a multi-planet system orbiting the star HR 8799. The three planets were discovered the old-fashioned way: they were directly imaged! [A gallery of all known directly-imaged exoplanets is at the bottom of this post, in fact.] The star is young (30 - 60 million years old), so the three planets are also young, and still glow with the leftover heat of their formation. In the infrared, they're bright enough to be distinguished from their star in images.
And now, follow-up observations using the monster Keck 10-meter infrared telescope have revealed a fourth planet: HR 8799 e:
See it there? The star is the weird blob in the middle, with most of its light removed using techniques that allow the much fainter planets to be seen. The other three planets, labeled b, c, and d (the letter "a" is reserved for the star itself, and is usually just assumed) are obvious as well.
Some aspects of the planet are pretty easy to observe; given the distance (130 light years) to the star, we can measure the planet's orbital size off the image; it's about 2 billion kilometers out, a little bit closer in than the distance of Uranus from the Sun. It takes roughly 50 years for it to complete one orbit.
The most important characteristic of the planet is its mass, and that's not all that well known. With some planets we can measure the gravitational tug of the planet on the star and deduce its mass from that, but that won't work here (the orbital period is too long, and it's easier if the plane of the orbit is edge-on to us, instead of the nearly face-on orbits of HR 8799's planets). So we have to rely on models using the physics of how a planet cools after it forms; how bright it is now depends on its mass and its age. More massive planets glow more brightly than lighter-weight ones, and they fade as they age and cool. The problem is, we don't know the age of the star well enough to nail down the planet's mass; if it's 30 million years old the planet is 5-10 times the mass of Jupiter; if it's 60 million years old it's 7 - 13 Jupiter masses. A good guess is probably about 7 - 10 times Jupiter. That's pretty firmly in the planetary mass range; if it were 13 or more it would be called a brown dwarf.
The thing is, it's not clear a planet of that mass can form that close to the star. We don't know everything there is to know about how planets form, but the current thinking is that something like this can form far more easily farther out, and then over millions of years it migrates closer to the star. There are a couple of ways this can happen; for example, there can still be lots of dust and junk orbiting the star leftover from its formation, and as the planet plows through this stuff it loses energy and gradually falls toward to the star. Interactions with other planets can also move it closer to the star as well.
The cool thing is, cases like HR 8799 e help us test our models, showing us where the holes are and making them better. And since HR 8799 is now known to host a full-blown solar system full of massive planets that can tug on each other, it's a fantastic testing ground for our understanding of the complex physics involved with making planets. And even better: over time we can watch as these planets physically move around their parent star, and we'll get even better data to feed our models. It's pretty amazing that we can see planets orbiting other stars at all, let alone learn so much from them.
Tip o' the deuterium limit to Universe Today.
[Below is a gallery of exoplanets that have been directly imaged using telescopes on ground and in space. Click the thumbnail picture to get a bigger picture and more information, and scroll through the gallery using the left and right arrows.]