Since the mid-1990s, when the first planet around another star was discovered, astronomers have been collecting exoplanets. In a new study, astronomers have classified these planets and found that most of the planets found to date are divided in size into two groups: rocky terrestrial planets and larger mini-neptune. The team used data from NASA's Kepler mission and Keck observatory.
In fact, the study shows that our Galaxy favors two types of planets: rocky worlds 1.75 times the size of Earth, and mini-neptunes covered with gas, which are 2 to 3.5 times larger than Earth. The Milky Way rarely forms planets with sizes between these two groups.
The diagram shows the number of small exoplanets per 100 stars, depending on their size in relation to the Earth. Credit: NASA / Ames / Caltech / University of Hawaii (BJ Fulton)
“There are no planets in the solar system with the size between Earth and Neptune. One of Kepler's greatest surprises is that almost every star has at least one planet larger than Earth but smaller than Neptune. We would love to know what these mysterious planets are and why we don't have them,”says Eric Petigura, co-author of the study at the California Institute of Technology (USA).
Kepler searches for distant worlds using the transit method. The magnitude of the eclipse correlates with the size of the planet, but in order to accurately determine its size, it is necessary to measure its star. A team of scientists from the California Institute of Technology, along with colleagues from several institutions, took a closer look at the size of the planets using the Keck Observatory. They have spent years collecting spectral data on stars with 2000 planets from the Kepler catalog. Spectral data provided accurate measurements of the size of stars, which in turn allowed researchers to determine the size of the exomers orbiting them.
With new data from the Keck Observatory, the researchers were able to determine the size of 2,000 planets 4 times more accurately. When scientists sorted them out, they were surprised to find a striking gap between groups of rocky lands and mini-Neptune.
The reason for the gap is not clear, but scientists have come up with two possible explanations. The first is based on the idea that nature prefers to create planets roughly the size of the Earth. Some of these planets, for reasons not yet fully understood, end up capturing enough gas to "cross the line" and become gaseous mini-neptunes.
The diagram shows how Earth-sized exoplanets and mini-neptune are formed. Credit: NASA / Ames / Caltech / University of Hawaii (BJ Fulton)
Promotional video:
“Mini neptunes are like rocks with large balls of gas around them. The hydrogen and helium in the ball do not contribute to the mass gain of the entire system as a whole, but they make a huge contribution to the volume, making the planets much larger in size,”continues Eric Petigura.
The second possible reason why the planets do not have intermediate sizes is associated with the loss of gas, or rather burning when exposed to radiation from the host star. Both scenarios likely explain the planetary size gap. “In the future, researchers plan to study the content of heavy elements in these planets in order to find out more about their composition. We live in the golden age of planetary astronomy because we find thousands of planets around other stars. We are currently working on understanding the composition of mini-neptune. This should explain why these worlds form so often in the orbits of other stars and why they do not exist around the Sun,”concluded Eric Petigura.
Roman Zakharov