Astronomers have discovered a six-pack of planets, formed at least 4 billion years ago and remarkably unchanged since, orbiting a nearby sun-like star. The new planets, described in a paper published Wednesday in the journal Nature, could provide a breakthrough in the understanding of how planets form and why there are so many between the sizes of Earth and Neptune, a class known as "sub Neptunes" that is astoundingly common in our galaxy.
These newfound worlds are hot, gassy and unlikely to be pleasant places to visit. Their cozy orbits around the parent star mean they are not in what astrobiologists consider the "habitable zone" of a planetary system. The hunt for Earth 2.0 goes on.
But what makes these planets unusual, in addition to their large number, is that they are locked into a resonance with one another as they orbit the star. One planet, for example, will make precisely three orbits while an adjacent planet makes two.
"These resonant chains are very rare in nature," lead author Rafael Luque of the University of Chicago said in a webinar Tuesday with reporters.
This striking reminder that mathematics governs the universe comes with another implication, which is that these six planets have been in a stable, predictable, two-by-three orbital pattern since they were formed at least 4 billion years ago. Most planetary systems, including our own, aren't like that.
The resonant orbits of these planets are consistent with the idea that this system has been free of any major disturbance - say, a catastrophic impact, or the close passage of another star - for billions of years. In this scenario, the planets formed along with their parent star from a cloud of gas and dust and relatively quickly found their resonant orbits. And then nothing exciting happened to change that.
This unusual orbital pattern is prominent in the grabby title of the new paper: "A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067."
"Occasionally, nature reveals an absolute gem," Sara Seager, a professor of planetary science at MIT and a co-author of the new paper, said in an email. "HD 110067 is an immediate astronomical Rosetta stone - offering a key system to help unlock some mysteries of planet formation and evolution."
The new paper, written by more than 150 scientists from 12 nations, describes the planetary system of HD 110067, a star in our galaxy. Located in the Coma Berenices constellation, it is not visible with the naked eye.
Still, it's only 100 light-years away, which means it is in our neighborhood, in a suburb of the Milky Way galaxy. That proximity to Earth makes it bright compared to many other stars previously known to have planetary systems. It is 10,000 times brighter, for example, than Trappist-1, a red dwarf star that also has an intriguing swarm of rocky planets.
Starlight is valuable currency for astronomers, who can study that light for clues to the presence of planets that are otherwise invisible amid the glare of the star. When a planet passes across the face of a star as seen from a telescope - an event known as a transit - the starlight will dim, commensurate with the dimension of the planet.
Astronomers can then employ a second technique to look for periodic wobbles in the starlight as an orbiting planet and the star interact gravitationally. Pairing these methods, astronomers can get an estimate of a planet's size and density. Further investigation can potentially detect the molecular composition of an atmosphere, if there is one.
Astronomers found the first couple of planets orbiting HD 110067 in 2020 using NASA's Transiting Exoplanet Survey Satellite (TESS), which scans the whole sky looking for subtle signals of planets. The planetary roster filled out in 2022 during another set of observations by TESS and by a European Space Agency satellite known as CHEOPS (for "CHaracterising ExOPlanets Satellite"), which has the ability for more targeted observations.
The new planets are called "sub Neptune" because they're bigger than the close-in, rocky worlds of our solar system, such as Earth and Venus, but not as big as the ice giants Neptune and Uranus. They range from two to three times the diameter of Earth. The innermost planet orbits the star in just nine days, while the outermost makes that journey in 54. There could be other planets in the system that remain undetected.
For some reason, the universe is lousy with sub-Neptune planets, which is one reason the new system is so exciting for astronomers.
"With six major planets, its architecture is intriguing," Knicole Colon, a NASA astrophysicist and exoplanet expert, said in an email. "These planets are likely not going to support life, as they are all likely too warm and too large. But still the whole sub-Neptune angle is the intriguing part, [because] we don't yet know why our solar system does not have one."
It's an open question whether the universe simply favors planets in this size, or if our detection methods are skewing the results. Small, rocky worlds like ours, orbiting at a comfortable distance from an old, calm star like our sun, are hard to find. They are less likely to transit the face of the star as seen from Earth, and they have minimal gravitational effects on the star's motion.
The discoverers of the new planets said there is evidence that they have atmospheres, based on their density. But, Luque noted, "we don't know much about them. We don't know what they are made of."
We may know more soon. This new planetary system will get a close look from NASA's James Webb Space Telescope, which orbits the sun about a million miles from Earth and is designed to glean information about the atmospheres of exoplanets.