By Will Dunham
WASHINGTON (Reuters) – In our solar system, little rocky Mercury is the planet orbiting closest to the sun, perpetually fried by solar radiation seven times more intense than what we experience on Earth.
Astronomers using data obtained by NASA’s now-retired Kepler space telescope have identified seven planets orbiting a star in our Milky Way galaxy, with all of them suffering the wrath of their star – radiant energy – even more brutally than Mercury. This is the second-most planets so far discovered around any star beyond our solar system.
All seven are larger than Earth, the biggest of our solar system’s four rocky planets, but littler than Neptune, the smallest of our solar system’s four gas planets. All of them have orbits closer to their star, called Kepler-385, than Mercury’s average distance to the sun.
“All of the planets are ‘fried’ more intensely than any planet in our solar system,” said astronomer Jack Lissauer of NASA’s Ames Research Center in California, lead author of the study set to be published in the Journal of Planetary Science and currently posted on the arXiv research site.
Scientists have to date identified more than 5,500 exoplanets – planets outside our solar system – and spotted hundreds of stars with multiple exoplanets. But Kepler-385’s collection of seven exoplanets is topped only by the eight known to orbit a star called Kepler-90. One other star, TRAPPIST-1, is known to have seven. Our solar system has eight planets.
The Kepler space telescope, NASA’s first planet-hunting mission, was retired in 2018. It detected exoplanets by observing small dips in a star’s brightness when a planet crosses in front of it from our vantage point.
The new study catalogs roughly 4,400 planets spotted by the telescope from its 2009 launch to its retirement. Scientists continue to analyze its data, as evidenced by the identification of Kepler-385’s population of exoplanets.
The study further illustrates that there are lots of different kinds of planetary systems – and many probably do not closely resemble our solar system. There almost certainly are planetary systems with more than eight, but telescopes so far have not been sensitive enough to do well detecting smaller exoplanets.
The star Kepler-385 is about 10% larger in diameter and mass than our sun, while being somewhat more luminous and slightly hotter. It is located about 5,000 light years from Earth. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).
The smallest of its seven planets – 20% larger than Earth – orbits closest to the star, at a distance of a little more than 4% of the distance between our planet and the sun. The next planet is about 20% larger than the innermost planet.
“Both of them are likely to be rocky, and tidally locked, showing the same face to their star all the time, as the moon does to Earth,” Lissauer said. “This makes them especially hot near the point closest to the star. But as any atmosphere is likely to long ago have boiled away, their hemispheres facing away from the star are perpetually dark and extremely cold.”
Most of the other planets are about 2.4 times larger than Earth.
“All likely have thick atmospheres, and are hot everywhere on their surfaces, which may be well below their cloud tops,” Lissauer said. “The outer planet orbits at about 40% of the Earth-sun distance. Its distance is slightly less than the average distance between the sun and Mercury.”
In the search for life beyond Earth, these planets are not promising candidates.
“The chance of life on any of these seven planets is indeed pretty remote,” Lissauer said. “There may well be additional planets orbiting farther from the star that we don’t know about because they are more difficult to detect. In particular, if there were an Earth-sized planet in the system at the Earth-sun distance, we would not have detected it.”
