NASA's Kepler probe spots youngest exoplanet ever

Scientists, using NASA's Kepler Space Telescope, have discovered the youngest fully formed exoplanet ever detected - only 5 to 10 million years old - that may help better understand how planets form.

The planet, K2-33b, is slightly larger than Neptune and whips tightly around its star every five days. It is one of the very few newborn planets found to date.

The discovery was made using NASA's Kepler Space Telescope and its extended K2 mission, as well as the W M Keck Observatory on Mauna Kea in Hawaii.

"Our Earth is roughly 4.5 billion years old. By comparison, the planet K2-33b is very young. You might think of it as an infant," said Trevor David, graduate student of California Institute of Technology (Caltech) in the US.

Planet formation is a complex and tumultuous process that remains shrouded in mystery. Astronomers have discovered and confirmed roughly 3,000 exoplanets so far.

However, nearly all of them are hosted by middle-aged stars, with ages of a billion years or more.

For astronomers, attempting to understand the life cycles of planetary systems using existing examples is like trying to learn how people grow from babies to children to teenagers, by only studying adults.

"The newborn planet will help us better understand how planets form, which is important for understanding the processes that led to the formation of Earth," said Erik Petigura of Caltech.

K2-33 system​ and planet K2-33b compared to planets from our solar system.

The first signals of the planet's existence were measured by the Kepler telescope's camera, which detected a periodic dimming of the light emitted by the planet's host star, a sign that an orbiting planet could be regularly passing in front of the star and blocking the light.

Data from the Keck Observatory validated that the dimming was indeed caused by a planet, and also helped confirm its youthful age.

Infrared measurements from NASA's Spitzer Space Telescope showed that the system's star is surrounded by a thin disk of planetary debris, indicating that its planet-formation phase is wrapping up.

Planets form out of thick disks of gas and dust, called protoplanetary disks, that surround young stars.

"Initially, this material may obscure any forming planets, but after a few million years, the dust starts to dissipate," said Anne Marie Cody, from the NASA's Ames Research Centre.

A surprising feature in the discovery of K2-33b is how close the newborn planet lies to its star. The planet is nearly 10 times closer to its star than Mercury is to our Sun, making it hot.

There are two main theories that may explain how K2-33b wound up so close to its star, researchers said. It could have migrated there in a process called disk migration that takes hundreds of thousands of years.

The planet may have also formed "in situ" - right where it is, researchers said.

The findings were published in the journal Nature.