'Teenage' Jupiter may hold the secret of how planets form

In the past 20 years, thousands of planets have been discovered orbiting other stars. Far from resembling families of planets like Earth and its companions, most of these discoveries have made our solar system look like the odd one out.

But now astronomers have announced a new exoplanet that looks surprisingly familiar. The exoplanet, 51 Eridani b, looks a lot like Jupiter – or at least the way we think Jupiter looked when it was much younger. Studying this juvenile version of our familiar neighbour will help us to unlock Jupiter’s past and find out more about the circumstances of its birth.

Bright young thing

The newly discovered exoplanet is a gas giant in orbit around a star 96 light years away. The star and its planetary system are estimated to be just 20m years old, less than a hundredth of the age of our solar system. This means that while Jupiter is a fully-grown planet, 51 Eridani b is still a teenager.

The youthfulness of 51 Eridani b was the key to its discovery. The planet has barely had time to cool down from its formation, which means that it is still bright enough to be directly imaged. Using the Gemini Planet Imager, a new instrument on the 8-meter Gemini South Telescope in Chile, an international team of astronomers was able to carefully block out the light from the parent star and spot the planet. The results have been published in the current issue of the journal Science.

Discovery image of the planet 51 Eridani b, taken in the near-infrared with the Gemini Planet Imager on December 18 2014. The bright central star has been mostly removed to enable the detection of the exoplanet.J. Rameau (UdeM) and C. Marois (NRC Herzberg)

The authors of the study estimate that 51 Eridani b is 2.5 times further from its star than Jupiter is from the Sun, meaning that if it was located in our solar system, it would sit between Saturn and Uranus. The planet is roughly the same size as Jupiter but being much denser has at least twice the mass.

Show us what you’re made of

By studying how the amount of light emitted by the planet varies with colour (or wavelength), scientists are starting to learn about the planet’s composition. “We already know that the atmosphere is rich in methane,” said Mark Marley, a co-author on the paper and an astronomer at NASA Ames Research Centre. “This planet has an atmospheric composition that is the most similar to our own Jupiter of any directly imaged planet.”

The biggest difference between the two planets is the temperature: 51 Eridani b is a sweltering 450°C, while Jupiter is much more frosty, at -150°C. But as time passes and 51 Eridani b gradually cools down, it will start to look more and more like our gas giant neighbour.

The exoplanet provides a glimpse back in time to how Jupiter might have looked when it was just a few million years old. Studying planets like this could be the key to unlocking the secrets of our own solar system.

“In the solar system we have been trying to understand the formation and evolution of giant planets just by studying them as they are today,” said Marley. “By studying young Jupiters, we are catching them closer to their birth and thus we hope to be able to see more clues about the details of their formation.”

How do planets form?

We know that planets are formed in the circular cloud of dust and gas that surrounds a newborn star, but the precise way in which this happens isn’t well understood. There are two main theories for the formation of gas giant planets: core-accretion, where material gradually clumps together into bigger and bigger pieces, and disc-instability, where there is rapid fragmentation into planet-size chunks as the circular cloud cools.

Artist’s impression of a planet forming within a gap in the dusty disc surrounding a young star.NASA/JPL-Caltech

Astronomers think that Jupiter formed through core-accretion, but until now, it had seemed like the odd one out. All of the young Jupiter-like exoplanets that had previously been discovered were too hot and bright for the core-accretion model to fit, suggesting that they formed via disc instability instead – 51 Eridani b is the first one that seems like it could have formed in the same way as Jupiter.

The first of many?

This is just the start, according to Marley. “Once we have more data we can begin to piece together the formation scenario for this planet and hopefully more planets that are yet to be discovered. Once we have a systematic view of many young giant planets we hope to understand planet formation much better than we do now.”

This is just the beginning for the GPI instrument, too, which is expected to make many more discoveries during its operational life. The young planets we hope it will find may hold the key to the history of our solar system. It may be only a teenager, but 51 Eridani b certainly has a lot to tell us.

Rohini Giles receives funding from the Royal Society.