An international team of astronomers has spotted a Neptune-sized exoplanet orbiting a very young star approximately 500 light-years from our own Solar System.

The newfound planet, named K2-33b, is 50% larger than Neptune, and its mass is less than 3.6 times that of Jupiter, with a true mass likely to be similar to that of Neptune.

K2-33b orbits its star once every 5.4 days and is only 5 – 10 million years old – as so still in its infancy in astronomical terms and when compared to Earth, which is 4.5 billion years old.

It also means it is the youngest, fully-formed exoplanet to have been identified and one of the few newborn exoplanets to have been discovered to date.

“At 4.5 billion years old, the Earth is a middle-aged planet — about 45 in human-years. By comparison, the planet K2-33b would be an infant of only a few weeks old,” said team member Trevor David, from the Cahill Center for Astronomy and Astrophysics at the California Institute of Technology.

“This discovery is a remarkable milestone in exoplanet science,” added team member Dr. Erik Petigura from the California Institute of Technology.

“K2-33b will help us understand how planets form, which is important for understanding the processes that led to the formation of the earth and eventually the origin of life.”

“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 team member Dr. Sasha Hinkley from the University of Exeter, UK.

“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.”

The first signals of K2-33b’s existence were measured by NASA’s Kepler space telescope during its K2 mission.

Kepler detected a periodic dimming in the light emitted by the planet’s parent star, K2-33, that hinted at the existence of an orbiting planet.

Observations from the W. M. Keck Observatory in Hawaii validated that the dimming was indeed caused by a planet, using both of the 10-m Keck telescopes: the HIRES instrument installed on Keck I and NIRC2 on the Keck II.

High resolution images obtained with Keck’s NIRC2 instrument in 2011 and 2016 were also used to confirm that there were no other nearby stars — either gravitationally bound to K2-33, or in the background but aligned by chance — that might be mimicking a planet transit signal.

“Astronomers know that star formation has just completed in this region, called Upper Scorpius, because roughly a quarter of the stars still have bright protoplanetary disks,” Mr. David said.

“The remainder of stars in the region do not have such disks, so we reasoned that planet formation must be nearly complete for these stars, and that there would be a good chance of finding young exoplanets around them.”

“We were able to study how frequently the light emitted from the star dimmed, and by how much, to determine the orbit and size of the new exoplanet,” Dr. Hinkley added.

The next step is to measure K2-33b’s mass and determine its density. These measurements will offer insights into the planet’s fate later in life — whether it will stay roughly the same size or if it will cool and contract.

The astronomers published their findings in the journal Nature on June 20, 2016.

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Trevor J. David et al. A Neptune-sized transiting planet closely orbiting a 5–10-million-year-old star. Nature, published online June 20, 2016; doi: 10.1038/nature18293