Scientists discover why Universe has very few planets double Earth’s size

Scientists discover why Universe has very few planets double Earth’s size
Study reveals how large gas-shrouded planets could shrink over billions of years, leaking away their atmosphere

Shrinking of large planets over billions of years likely explains the years-long mystery behind why there is a scarcity of planets close to double the Earth’s size in the Universe, say scientists in a new study.

研究者によると, including those from Flatiron Institute in the US, while planet-hunting missions have discovered thousands of worlds orbiting distant 出演者, the number of such exoplanets measuring between 1.5 and two times Earth’s radius is very rare.

の中に 調査, に発表されました The Astronomical Journal, the scientists say this ‘radius-gap’ – first discovered in 2017 – could be due to a process where large gas-shrouded planets called mini-Neptunes shrink over billions of years as their atmospheres leak away.

As these mini-Neptunes lose their gas, they “jump” the planet radius gap and become super-Earths, larger than our planet, the researchers explained.

この研究で, the scientists sorted exoplanets spotted by the Kepler スペース telescope into two categories – young and old – and reassessed the radius gap.

In the younger set they found that the least common planet radii were smaller on average than those from the older set, while the rarest size for younger planets was about 1.6 times Earth’s radius, それは 1.8 times Earth’s radius at older ages.

言い換えると, they found that the radius gap among exoplanets was emptier than previously thought.

Based on the results, the scientists theorised that mini-Neptunes may likely shrink drastically over billions of years with their gas layers leaking away, leaving behind only a solid core.

The researchers believe this radius gap is the chasm between the largest-size super-Earths and the smallest-size mini-Neptunes that can still retain their atmospheres.

They said the process takes longer for larger mini-Neptunes — which become the largest super-Earths — but may not impact the largest gas planets whose gravity is strong enough to hold onto their atmospheres.

Earlier studies had theorised that the size of planets were generally imprinted at birth depending on how much gas there was nearby at the time of their formation to puff up their size.

Another theory had proposed that collisions with space rocks could have blasted away the thick atmosphere, preventing tinier planets from accumulating lots of gas – a mechanism that would take about 10 百万から 100 百万年.

With the current findings, the scientists are confident that the process usually takes billions of years, suggesting that the shrinking isn’t due to planetary collisions or an inherent trait of planetary formation.

“The overarching point is that planets are not the static spheres of rocks and gas we sometimes tend to think of them as. Some of these planets were 10 times larger at the starts of their lives,” Trevor David, study co-author and a research fellow at the Flatiron Institute, 声明で言った.