We're aware of a huge number of exoplanets now, but we've only seen about a dozen. Most of the rest have been detected through their effects on the light emitted by the stars they orbit. Some of the bodies we've imaged directly may not even be planets; they're so large that they may have briefly undergone fusion, which puts them in the brown dwarf category.

But astronomers have now imaged an object that very clearly falls into the planetary category, as it may be as little as four times the mass of Jupiter (if one could ever use "little" for a planet that size). Like all good scientific findings, it comes with a bit of mystery: we don't think a planet that size can form where this one orbits.

The exoplanet was discovered as part of a survey of young stars that are relatively close to the Sun. "Close to the Sun" simply means that we can obtain more light from them; "young stars" mean that any planets orbiting them will also be young. In the case of gas giants, that means they'll be hot, as they will still be radiating away the heat of their formation. One of the stars the survey imaged was HD 95086, which is about 300 light years from Earth. In addition to being young—previous estimates have placed the star at either 10 or 17 million years old—the star was already known to have a dusty environment, which could indicate a planet-forming disk.

The authors had two chances to image the star, once in 2012 and once earlier this year. The data was then heavily processed to remove as much of the light from the star as possible. When that was done, two objects remained in the image. Analysis of the images suggests that one of the objects is simply a background star that happens to be near HD 95086. But the second seems to be moving along with the star, implying it could be a planetary companion (the authors claim a three-sigma confidence in this identification).

The amount of infrared light emitted by the planet will be proportional to a combination of its age and size; to emit the same amount of light, a smaller planet would have to be younger. Based on the two estimates of the star's age, the authors calculate masses of either four or five times the mass of Jupiter, which makes the object the lowest-mass planet we've ever directly imaged. It appears to orbit at about 55 Astronomical Units, or roughly twice the distance between the Sun and Neptune.

The distance poses a bit of a problem. Gas giants like this one need to form quickly, before the star's energy starts driving the gas away from the proto-planetary disk. To do that, they need to form relatively close to the star, where solid material is present at high enough levels to quickly form a rocky core that's about 10 times the mass of Earth. The rocky core then needs to start grabbing gasses. However, that far out from the host star, there's simply not enough rocky material around to create the core. So even if there were enough gas where this thing orbits, it shouldn't have formed there.

There are a number of potential ways to explain this, the simplest being that the body formed closer to the star but was ejected from the inner regions of that exosolar system by interactions with other planets. It's also possible that a star-like formation mechanism may have produced this body through gravitational collapse.

Rather than focusing on the body's formation, the authors seem mostly interested in confirming that it really is a planet. The more recent observations suffered from bad weather, which limited the confidence the astronomers have in their identification. Another night with some good weather, they suggest, should be enough to really nail things down. And simply having more examples of directly imaged planets has its own value. As they write, "every single discovery has a tremendous impact on the understanding of the formation, the dynamical evolution, and the physics of giant planets."

The paper has not been published yet, but it has been made available by the European Southern Observatory.