"That looks interesting!" exclaimed a researcher, looking through data from a telescope. He had spotted something in a star system that many other telescopes had looked at before and missed. The statement marked the discovery of a new exoplanet in a young star system, only 100 light-years away. The exoplanet has a strong abundance of methane gas and other characteristics that are surprisingly similar to the biggest gas giant in our own Solar System, Jupiter. Scientists are hopeful that this exoplanet might give us additional clues about how our own dominant gas giant, and others, are formed.

The data came from the Gemini Planet Imager (GPI), a newly installed instrument on the Gemini South observatory in Chile, which was designed specifically to seach for planets. It blocks out the light of a star to see the image of any planets that might be rolling around it.  You can see the results from this study in Science.

This is the first discovery by this new instrument, and it showcases its superior planet-spotting ability; four other observatories looked at the star, 51 Eridani, before the GPI, and none of them saw this planet, dubbed 51 Eridani b. But its discovery was difficult even for the GPI, and only some key features of the exoplanet allowed it to be found.

"For [the GPI] to work, the planet has to be young right now; we can't see an old planet like Jupiter that's just reflecting starlight – we can only see the infrared emission that a young planet has," Bruce Macintosh from Stanford University, who led the research, told IFLScience. 

The newly discovered planet itself is a youngster at a mere 20 million years old. The scientists deduced, based on the brightness of the planet, that its mass is about twice that of Jupiter: the most massive planet in our Solar System at 317 times the mass of Earth. However, Macintosh said that this value is "very uncertain since it's based on theoretical models. We don't get to measure the mass directly." There is still uncertainty in the calculation since the planet is much younger than Jupiter, so there are more than four billion years of changes between them.

Regarding the birth of this planet, which could have implications for the birth of other gas giants, there are two main competing theories.

One is that there was a build-up of matter that formed the core first. The core was then powerful enough to suck in excess gas that was swirling around the Sun. Alternatively there could have been a sudden collapse of dust and gas that produced the planet in one swift motion. 

This planet is theoretically young enough that, given more tests, scientists could figure out how it formed and maybe even extend this knowledge to understand how Jupiter formed. 

Studying the light spectrum (the light frequencies emitted due to different atoms and compounds) of the planet has let the scientists see its chemical composition. Macintosh and his team were delighted when they saw the spectrum. "This planet has a beautiful methane signature," he said, which is a gas that scientists have been searching for since the first exoplanet was discovered. It has been seen in trace amounts on other planets, but nothing comparable to the levels on Jupiter, so to finally see it in this survey is encouraging. It indicates that the planet has a low temperature and is another clue that points to the this planet having a similar mass to Jupiter. 

The presence of this heavy Jovian planet leads to conjecture about other similarities that this star system may have to the Solar System. "There's certainly room for an Earth," around 51 Eridanus, Macintosh said. The young Jupiter planet is far enough away from its star that it wouldn't interrupt the growth of rocky planets nearer the star (even though it is speculated that Jupiter helped form the Earth). There is also a very light asteroid belt around this star, sparser than around other star systems, indicating that some of the matter may have gone into forming rocky planets closer to the star.

Macintosh reckons that a telescope big enough to photograph details on this planet, for example its clouds, is about "40 years away." He thinks that an array of telescopes in space covering around a kilometer (0.6 miles) would be capable of this detail. In the meantime, there's still GPI data from lots more stars to run a fine-toothed comb through. Hopefully more planet discoveries will come flooding in!

^{Central Image: Gemini Telescope in Chile.Wikimedia Commons (CC BY 3.0).}