WASHINGTON : They are the most common type of planet observed in our Milky Way galaxy - two to three times the diameter of Earth but smaller than Neptune, and orbiting closer to their stars than our solar system's innermost planet Mercury does to the sun.
Called "sub-Neptunes," they are absent from our solar system and their fundamental nature has remained a puzzle. But the discovery announced on Wednesday of six of them in synchronized orbits around a star about 20 per cent smaller in mass than the sun is giving astronomers hope that an answer could come soon.
The researchers determined that the six planets were in a rare condition called orbital resonance, with their synchronized orbits around the star apparently unchanged since they formed about 4 billion years ago. That indicates no chaotic event like a giant impact event has perturbed their orbits.
"The resonance aspect is really interesting - partly the mathematical beauty of it," said astronomer Hugh Osborn of the University of Bern in Switzerland, one of the authors of the research published in the journal Nature.
"The key thing about this system is its potential to unlock the secrets of these mysterious sub-Neptune planets, which we know so little about," Osborn added. "These are definitely not Earth-like planets."
Hundreds of sub-Neptunes have been discovered.
"What these sub-Neptunes are made of is an active topic of research in the field since there are multiple combinations of rock, water and atmospheric composition that can reproduce the bulk properties - mass, radius and density - of the planets," said University of Chicago astronomer Rafael Luque, lead author of the study.
Scientists have wondered whether sub-Neptunes might be rocky planets with thick atmospheres of hydrogen and helium gas, planets made of rock and ice bearing warm and water-rich atmospheres - or something else.
Earth, the largest of our solar system's four rocky planets, has a diameter of about 7,900 miles (12,750 km). Neptune, the smallest of its four gas planets, has a diameter of about 30,600 miles (49,250 km), roughly four times that of Earth.
The newly discovered sub-Neptunes range from 1.9 to 2.9 times Earth's diameter. All appear to possess a large atmosphere. They and their star are located around 100 light-years from Earth. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). Their star, called HD110067, is visible in Earth's ni...