Two planets have been discovered that are among the youngest ever observed, providing a new insight into how quickly planets can form around stars.

The first, orbiting a star 500 light-years from Earth, is called K2-33b. Discovered by NASA’s Kepler space telescope, its size is larger than Neptune in our own Solar System, but its orbit around its star is so close (0.05 the Earth-Sun distance) that its year lasts just 5.4 days. Its discovery is published in Nature.

At five times the size of Earth and twice the size of Neptune, the planet is dubbed a super-Neptune, and it is thought to be just 5 to 10 million years old, a blink of an eye in cosmic terms. For comparison, Earth is 4.5 billion years old.

“It is extremely rare to find a planet at this stage of its infancy, and gives us a unique opportunity to try and understand more about how all planets form and develop, including Earth,” said study co-author Dr Sasha Hinkley, a senior lecturer in Astrophysics and Astronomy at the University of Exeter, in a statement.

^{The K2-33 system compared to our own. NASA/JPL-Caltech}

Large planets in tight orbits like this (an animation is available here) have long puzzled astronomers, as it’s not clear how these planets end up so close to their stars. Theories range from the planets somehow forming in their close orbits, migrating from elsewhere in their system, or being captured from other stars.

“We would really like to know if this planet formed at its present location, or perhaps formed much farther from the star, and moved much closer in,” added Dr Hinkley. Its star still retains some of the original disk of dust and gas that gives rise to stars and planets, suggesting the system is still in its final stages of formation.

Most of the 2,000 or so planets we have found so far are middle-aged, with ages of billions of years or more. Thus, this world provides a snapshot into the early formation of planets, something that is not fully understood at the moment.

The same is also true for the second planet announced today, a hot Jupiter orbiting a young star that’s just 2 million years old. This discovery is also published in Nature.

The star is called V830 Tau, and it is located about 430 light-years from Earth. The planet discovered has an orbit that lasts 4.9 days, again located at 0.5 times the Earth-Sun distance. It was detected by observing the “wobble” in the host star as the planet swung around.

“Our discovery reveals that a giant planet can not only form quickly, but also end up extremely close to its sun soon after the star itself is born,” said co-author Elodie Hébrard, from York University, in a statement. “What’s more, the presence of a close-in giant planet so early in a star’s life is likely to have a profound influence on smaller, terrestrial planets that might form in its vicinity.”

^{Artist's impression of the planet around V830 Tau, which is just 2 million years old. Mark A. Garlick / markgarlick.com}

The finding suggests large planets can form early in a star’s life, and thus may have a crucial role to play in planetary formation and the positions of planets in systems. It also hints at an early migration of large planets from distant to close orbits in young stellar systems, something that has not been observed before.

Both planets are fascinating in their own right, and highlight how much we have to learn about planetary formation. What’s also becoming apparent is that our own Solar System is a bit of an anomaly. Many other systems have hot Jupiters; why doesn’t ours? That’s a question that cannot be fully answered yet.