Since its launch in 2009, NASA’s Kepler spacecraft (with some help from other telescopes) has discovered close to 3,000 planets. They all come in different sizes and orbits. Many of them are tidally locked to their host star. Some orbit further away. A few even sit in the habitable zone. But nearly all of them have one thing in common – age. Nearly every planet discovered so far is a billion years old or older.

That changes with K2-33b. This exoplanet is like many of the others spotted by Kepler. It’s a little bigger than Neptune and tightly orbits its host star once every five days. But there is one huge difference. Scientists believe the planet is just 5-10 million years old.

Here’s how Trevor David, the first author on the paper describing K2-33b, describes the exoplanet.

“At 4.5 billion years old, the Earth is a middle-aged planet – around 45 in human-years,” says David. “By comparison, the planet K2-33b would be an infant of only a few weeks old.”

Co-author Erik Petigura says the discovery will be huge for understanding planet formation. It’s an area of astronomy we just don’t know much about. What were the exact processes that created our solar system? NASA’s press release explains it best:

“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.”

Finding K2-33b

Finding an exoplanet isn’t a one telescope job. Kepler tends to get all of the accolades, but it usually needs some help to confirm the discovery. Hints of K2-33b were first seen by Kepler’s camera. It noticed the host star was dimming in regular intervals. This is the tell-tale sign of an exoplanet transiting (a planet moving in front of its star). Follow-up observations from the Keck Observatory in Hawaii confirmed the dimming was from a planet.

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The discovery of K2-33b’s tight orbit is a surprise. Here’s how the K2-33 system looks compared to our own. You can see how much tighter K2-33b’s orbit is compared to Mercury’s.

K2-33 system and solar system

The orbit itself isn’t a surprise. Kepler has discovered countless exoplanets orbiting their stars this close. But combine the close orbit with the apparent age of the exoplanet and astronomers are left scratching their heads. Astronomers aren’t sure how these massive planets end up so close to their stars. Some theories suggest the exoplanets move closer over a period of hundreds of millions of years. But that explanation doesn’t work with K2-33b’s age.

The researchers have two theories for K2-33b’s orbit. A process called disk migration could have shifted the exoplanet’s orbit closer in a shorter period. The simplest explanation is the planet formed right where it is.

“After the first discoveries of massive exoplanets on close orbits about 20 years ago, it was immediately suggested that they could absolutely not have formed there, but in the past several years, some momentum has grown for in situ formation theories, so the idea is not as wild as it once seemed,” said David.

Observations of K2-33b will continue

While young, K2-33b is fully formed. Infrared measurements from NASA’s Spitzer Space Telescope shows the host star is surrounded by just a thin disk of planetary debris.

NASA spitzer telescope

NASA’s Spitzer telescope during assembly.

Astronomers will continue to gather more data on the exoplanet. What is its mass? How dense is it? And more. These measurements will give clues into how the planet will evolve in the future.

“In the last 20 years, we have learned that nature can produce a staggering diversity of planets—from planets that orbit two stars to planets that complete a full orbit every few hours,” Petigura says. “We have much to learn, and K2-33b is giving us new clues.”

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Image credits: NASA

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