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Shrinking planets could explain the mystery of the cosmic lost world.

The mysterious concept of exoplanets

By studying data from the Kepler Space Telescope, the Flatiron Institute researchers found that billions of years of planetary shrinkage may explain a long-lasting mystery: The scarcity of planets is about twice the size of Earth.

There has been progress in the case of the disappearing planets

As a planetary mission has discovered thousands of Earth orbiting distant stars. But there is also a serious shortage of exoplanets measuring between 1

.5 and twice the radius of the Earth. That is, the middle ground between rocky super-Earths and large gas-covered planets, called mini-Neptunes Since discovery In 2017, scientists investigated why there were only a few medium-sized celestial bodies.

New clues arise from a new way of looking at information. A team of researchers led by Flatiron Institute’s Trevor David investigated whether the radial gap changes with the age of the planet. They divide the exoplanets into two groups – children and adults – and re-evaluate the gaps. The least common planetary radius from the younger set is smaller, on average, less common than the least common common radius from the older set. While the rarest size for the youngest planet is about 1.6 times the Earth’s radius, it’s about 1.8 times the Earth’s radius as it ages.

The implications for the researchers are that some smaller Neptune stars have shrunk dramatically over billions of years as their atmospheres leak out to only a stable core. With the loss of gas, Mini Neptune “jumps” along the space of the planet’s radius and becomes super-earth. Over time, the radius gap changes as the smaller, larger Neptune jumps change. It’s a bigger and bigger super-earth. In other words, the gap is the gap between the largest super-earth and the smallest Neptune that retains the atmosphere. The researchers report their findings on May 14, 2021 in Astronomical Journal.

Exoplanet Radius Gap Infographic

Infographic explaining exoplanet radius space.Credit: Simons Foundation.

“The overarching issue is that planets are not the constant spheres of rock and gas that we sometimes think they are,” said David, a researcher at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City. In a previously proposed atmospheric depletion model, “some of these planets were 10 times larger at the start of life.”

The findings give credibility to two previously proposed suspects in this case: residual heat from planet formation and intense radiation from the host star. Both phenomena add energy to the planet’s atmosphere, causing gas to escape into space. “Both impacts may be important,” David said, “but we will have to use more complex models to say that each effect has. Help and when ”in the life cycle of the planet.

The paper’s co-authors are fellow CCA researcher Gabriella Contardo, CCA co-author Ruth Angus, CCA co-author Megan Bedell, CCA researcher Daniel Foreman-Mackey. And CCA invited researcher Samuel Grunblatt.

The new study uses data collected by the Kepler spacecraft, which measures light from distant stars when Exoplanets Moving back and forth between the star and the Earth, the light observed by it dims. Astronomers can estimate the size of exoplanets by analyzing how fast the planets orbit the star, the size of the star, and the dimming extent, astronomers can estimate the size of the exoplanet. These analyzes eventually led to the discovery of the radial gap.

Computer simulations of how the planetary size distribution changes as planetary systems age. The radius gap is evident at around twice the radius of the Earth, although it depends on the planet’s orbital period. Evidence has shown that the gap has changed over time, as does the gas-covered mini.Neptune The planets have lost their atmosphere, leaving behind a solid super-Earth. One of the planets that are in the process will be highlighted. (Shown as the core with the atmosphere) with size variation plotted on the right. Credit: Animation by Erik Petigura (UCLA); Simulation by James Owen (Imperial College London)

Previously, scientists have proposed some possible mechanisms for creating spaces, with each process taking place at different times. Some believe that gaps occur during planetary formation when certain planets form without enough gas close to their size to inflate. In this scenario, the planetary radius and radius space are imprinted from birth. Another hypothesis is that collisions with space rocks can blow off the planet’s thick atmosphere and prevent smaller planets from accumulating large amounts of gas. This effect mechanism will take approximately 10 million to 100 million years.

Other possible mechanisms require more time. One of the proposals is that intense X-rays and ultraviolet rays from the planet’s host stars will remove gases over time. This process is called photoevaporation It could take less than 100 million years for most of the planets. But some of them can take billions of years. Another suggestion is that the residual heat from the formation of planets will slowly add energy to the planet’s atmosphere, causing the gas to escape billions of years into space.

David and his colleagues begin an investigation, looking more closely at the gap. Measuring the size of stars and exoplanets can be tricky, so they clear the data to include only planets for which their diameter is known. Processing of this data revealed more gaps than previously thought.

The researchers then sorted the planets according to whether they were 2 billion years younger or older (for comparison, Earth is 4.5 billion years old) .Because stars and planets form together, each planet’s age is determined by age. Of the star

The results of the study suggest that the tiny Neptune cannot store gas. Billions of years ago, the gas was wiped out behind most of the solid super-earths. The process took longer for the smaller, larger Neptune to become the largest super-earth. But it won’t affect the most massive gas planets, which have a strong enough gravity to hold onto the atmosphere.

The fact that radial gaps evolved over billions of years suggests that the culprit is not a planetary collision or a planetary formation. The remaining heat from the planet’s interior will gradually remove the atmosphere, which is a good thing. But intense radiation from the parent star can play a role too, especially early on. The next step is for scientists to better model how planets evolve to find out which explanations play a bigger role. That could mean further complexity considerations such as the interactions between the bird’s atmosphere and the magnetic field of the planet or the magma ocean.

Reference: “Evolution of Exoplanet Size Distribution: Massive Super-Earth Formation Over Billions of Years” by Trevor J. David, Gabriella Contardo, Angeli Sandoval, Ruth Angus, Yuxi (Lucy). Lu, Megan Bedell, Jason L. Curtis, Daniel Foreman-Mackey, Benjamin J. Fulton, Samuel K.Grunblatt, and Erik A. Petigura May 14, 2021. Astronomical Journal.
DOI: 10.3847 / 1538-3881 / abf439

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