Global Statistics

Evidence that planets shrink


An intriguing class of Neptune-sized planets shrink as they are bombarded with intense light from their host stars for billions of years.

After centuries of studying the planets within our solar system, astronomers have wondered how planets form and evolve to become the ones we observe them today. One of the most surprising finds of the last decade was the discovery of a new branch in the planetary “family tree”, which separates the planets slightly larger than Earth (super-Earths) from those slightly smaller than Neptune (sub-Neptunes).

However, it is unclear how these different sized planets formed, as our observations they are just a snapshot of billions of years of useful life for each individual planetary system.

Astronomers cannot see the evolution of planets in real time, so they analyze populations of planets to infer how they form and evolve. In fact, using observations from NASA’s Kepler and ESA Gaia missions, University of Hawaii Institute of Astronomy (IfA) graduate student Travis Berger and his team have discovered another piece of the puzzle of the formation and evolution of the planets: As planets are bombarded with intense light from their host stars, they gradually lose their atmospheres over billions of years.

“The loss of planetary atmospheres on timescales of billions of years shows that these planets lose mass, even in old age,” Berger explained. it’s a statement. “One of our main discoveries is that the size of the planets shrinks on longer timescales than previously thought.”

NASA’s Kepler mission searched for planets by staring at a patch of sky near the constellation Cygnus for about four years, detecting small, regular dips in brightness from hundreds of thousands of stars within our Milky Way. The size of a dip corresponds to the relative size of the planet compared to its host star. Therefore, to determine the actual size of a planet, it is first necessary to measure the size of the star.

ESA’s Gaia mission provided an essential ingredient for measuring the size of the stars that host Kepler planets: parallax. Human eyes use parallax to measure distances to objects, giving us depth perception. Similarly, astronomers use parallax for astronomical-scale depth perception to measure distances to stars, which in turn helps measure the size of stars. Distance information is needed to distinguish between a smaller, closer star and a larger, more distant star. The combination of stellar sizes and colors also allows astronomers to determine the relative ages of the stars.

The University of Hawaii team used Gaia’s restrictions on the size of stars to review estimates of planet sizes and combined them with stellar color data to determine the ages of stars that host planets. They then compared the effects of star age on more than 2,600 planets detected by Kepler. Some planets, especially those that receive more than 150 times the light that the Earth receives from the Sun, they lose their atmospheres for a billion years, as they are flooded with heat and light from the host star.

“While astronomers have long predicted that planets should shrink in size as they age, we didn’t know if this can happen on timescales of billions of years. We do it now,” says Berger. “The fact that we see changes in the size of planets on timescales of billions of years suggests that there is an evolutionary path, where highly illuminated subneptune-size planets go on to become super-Earth-size planets. “

In the future, similar work could be done on planets discovered by the NASA K2 and TESS missions to solve the timescale for the loss of atmosphere more precisely.

The study is published in The Astronomical Journal.

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