2021-05-31

Breakthrough progress: research finds that shrinking planets can explain the mystery of the missing world in the universe

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Breakthrough progress: research finds that shrinking planets can explain the mystery of the missing world in the universe

New clues arise from a new way of looking at data. A research team led by Trevor David of the Flatiron Institute investigated whether the “radius gap” will change as the planet ages. They divided the exoplanets into two groups-young and old-and reassessed the gap. They found that the least common planets in the young group had smaller radiuses on average than the least common planets in the older group. Although the rarest size of a young planet is approximately 1.6 times the radius of the Earth, at an older age it is approximately 1.8 times the radius of the Earth.

The researchers proposed that this means that some mini-Neptunes have shrunk dramatically in billions of years, because their atmosphere has disappeared, leaving only a solid core. Due to the loss of gas, these mini-Neptunes “jumped” over the gap in planetary radius and become super-Earths. Over time, the radius gap will change as the larger and larger mini-Neptune jumps and become larger and larger “super-Earths.” In other words, this gap is the gap between the largest-sized “super-Earth” and the smallest-sized mini-Neptunes, which can still retain their atmospheres. The researchers reported their findings in the Astronomy Journal on May 14, 2021.

Breakthrough progress: research finds that shrinking planets can explain the mystery of the missing world in the universe(1)

From the Flatiron Institute of New York City Computational Astrophysics Researcher David of the Center for International Studies (CCA) said: “The first point is that planets are not static spheres of rocks and gases that we sometimes tend to think. In some of the atmospheric loss models proposed previously, some of these planets need to be at the beginning of their lives. 10 times larger.”

These findings confirm the two doubts raised before: the heat left by the formation of the planet and the intense radiation from the host star. Both of these phenomena increase energy in the planet’s atmosphere, causing the gas to escape into space. “David said: “Perhaps both of these effects are important, but we need more complex models to illustrate how big their respective contributions are and when they are in the life cycle of a planet. ”

This new study uses data collected by the Kepler spacecraft, which measures light from distant stars. When an exoplanet moves between a star and the Earth, The observational light from the star will dim. By analyzing the speed of the planet orbiting its star, the size of the star, and the degree of dimming, astronomers can estimate the size of the exoplanet. These analyses eventually led to the discovery of the “radius gap” .

Scientists have previously proposed some potential gap generation mechanisms, and each process occurs on a different time scale. Some people believe that the gap occurs during the formation of planets, while some planets did not form at that time. There is enough nearby gas to expand its volume. In this case, the radius of the planet, and therefore the radius gap, will be confirmed at birth. Another hypothesis is that collisions with space rocks may blow up the planet’s The thick gas layer prevents smaller planets from accumulating large amounts of gas. This collision mechanism takes about 10 million to 100 million years.

Breakthrough progress: research finds that shrinking planets can explain the mystery of the missing world in the universe(2)

Other potential mechanisms require more time. One suggestion is that the intense X-ray and ultraviolet radiation from planetary host stars will over time Stripping. This process is called photoevaporation, and it takes less than 100 million years for most planets, but it may take billions of years for some planets. Another suggestion is that planets form The residual heat at time will slowly add energy to the planet’s atmosphere, causing the gas to escape into space over billions of years.

David and his colleagues carefully observe the gap itself They started their investigation. Measuring the size of stars and exoplanets is very tricky, so they cleaned up the data to include only those planets whose diameters are believed to be known. This data processing revealed a larger than previously thought The researchers then classified the planets according to whether they were younger or older than 2 billion years ago. Since a star and its planets were formed at the same time, they determined the age of each planet based on the age of the star.

Breakthrough progress: research finds that shrinking planets can explain the mystery of the missing world in the universe(3)

The research results show that the smaller Mini Neptune cannot maintain its gas. After billions of years, the gas is stripped, leaving a mostly solid “super-Earth.” This process takes longer for the larger Mini Neptune Time, but will not affect the largest gas giant planets, their gravitational force is enough to maintain their atmosphere.

Breakthrough progress: research finds that shrinking planets can explain the mystery of the missing world in the universe(4)

The fact that the “radius gap” has evolved over billions of years shows that The culprit is not an inherent feature of planetary collisions or planetary formation. David said that the gradual peeling of residual heat from the planet’s atmosphere is a good opportunity, but strong radiation from the parent star may also play a role, especially in the early days. The next step is for scientists to better simulate how planets evolve to determine which explanation has played a greater role. This may mean considering additional complexities, such as the interaction between the new atmosphere and planetary magnetic fields or magma oceans.