If three’s a crowd, then six makes a chorus.
That’s the case at least for a rare six-planet solar system whose discovery has wowed astronomers, even in an era when detecting planets around distant stars has become a common occurrence.
All six of the planets are larger than Earth but smaller than Neptune. That likely means that they are small gaseous worlds surrounded by extended atmospheres of hydrogen and helium – not ideal for life as we know it and too hot because of their close proximity to the star they orbit.
The planetary sextuplet is a spectacular scientific find, astronomers say, because all six members of the system circle their star in a harmonious pattern with orbital periods that occur in simple, whole-number ratios, similar to those that dictate which notes sound good together in a standard musical scale.
For example, the innermost planet orbits three times for every two orbits completed by its nearest neighbour. That planet, in turn, orbits three times for every two orbits of the next planet out and so on in ratios of 3:2 or 4:3. By the time the outermost planet has completed one orbit, the innermost has gone around the star precisely six times.
This mathematical arrangement – an example of what is called orbital resonance – is reinforced by the gravitational forces between the planets and predicted by theories of planet formation. But astronomers have found that most solar systems are unable to maintain such a delicate pattern. Eventually, other forces and perturbations will come along to knock them out of sync.
In this case, the six planets seem to have held to their current pattern for billions of years, offering researchers the chance to work backward and uncover how such a system might come to be.
“This system is poised to become a test bed for a controlled experiment to study how planets form, how planets evolve,” said Rafael Luque, an astronomer at the University of Chicago and lead author of a study describing the find. “We are going to learn a lot.”
The study was published Wednesday in the journal Nature.
Dr. Luque said in a news briefing that the solar system is exceptional because, apart from our own sun, the star is the brightest known with so many planets around it. Designated HD11006, the star is located about 100 light-years away in the constellation Coma Berenices. If it were the same distance as our sun, it would appear only 20 per cent dimmer.
Since planets do not shine by their own light but can only reflect or block the light of the stars they orbit, a bright star offers a better opportunity to study a planetary system.
None of the planets have been seen directly. Their discovery was only possible because of a fluke of geometry. Since their orbital plane is roughly in line with astronomers’ line of sight from Earth, the planets appear to repeatedly cross in front of their star, creating a complex series of mini-eclipses or transits.
The first planet, designated HD11006b, was discovered with NASA’s TESS satellite in 2020 by using the transit method. But the data hinted at more planets. When TESS was able to view the system again in 2022, some additional planets were detected. But it wasn’t until Dr. Luque and his colleagues were able to gain time on CHEOPS – a European Space Agency satellite intended for long-duration studies of exoplanet targets (planets beyond our solar system) – that all six with their interrelated orbital periods were confirmed.
Follow-up observations that reveal how the star wobbles in response to the gravitational pull of the planets then yielded their individual masses.
The find is similar to another solar system called Trappist-1, which contains seven Earth-sized rocky planets locked in various orbital resonances. What makes the new system different is that the planets are bigger. All six fall into a category called “sub-Neptunes,” which are unknown in our solar system but common in others.
Because planets of that size are not well understood, the system represents a unique opportunity to study multiple examples that emerged in similar conditions.
“How do such planets even form? We have hypotheses, but to sharpen them we need to study the atmospheres of these planets,” said Nicolas Cowan, an astronomer who specializes in exoplanets at McGill University and who was not involved in the discovery. “Having six of them transiting the same star is a gold mine.”
Transiting planets are ideal for such studies because as they pass in front of their star, their atmospheres filter the light, revealing details about the planets’ compositions.
Researchers in Canada have already used NASA’s James Webb Space Telescope to conduct an atmospheric study of one planet circling in Trappist-1. The new find gives astronomers using JWST a chance to compare and contrast the two solar systems.
“Expect astronomers to rapidly jump on this system,” said René Doyon, director of the Trottier Institute for Research on Exoplanets in Montreal and a principal investigator on the Webb telescope.
He said some of the planets could be “water worlds,” meaning a significant fraction of their bulk mass consists of water.
“One could certainly qualify the Trappist-1 system as nature’s gift for studying rocky planets,” Dr. Doyon said. “Well, here’s the best gift ever for studying mini-Neptunes.”
Dr. Cowan added that the newly discovered system was tailor-made for study by a new exoplanet-dedicated mission that he is participating in. Called Ariel, the European Space Agency is set to launch it in 2029.
The Globe and Mail, November 29, 2023