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Two families of exoplanets redefine the appearance of planetary systems

Two families of tight exoplanets are pushing the boundaries of the appearance of a planetary system. New studies on the composition of worlds orbiting two different stars show a wide range of planetary possibilities, all different from our solar system.

“When we study multiplanet systems, there is simply more information stored in these systems” than any other planet by itself, says geophysicist Caroline Dorn of the University of Zurich. The study of the planets together "tells us about diversity within a system that we cannot obtain by looking at individual planets."

Dorn and colleagues studied an old favorite planetary system called TRAPPIST-1, which houses seven Earth-sized planets orbiting a small faint star about 40 light-years away. Another team studied a recently identified system called TOI-178, which has at least six planets – three already known and three recently discovered – that surround a bright, hot star about 200 light-years away.

Both systems offer planetary scientists an advantage over the more than 3,000 other families of exoplanets discovered to date: the seven planets in TRAPPIST-1 and the six in TOI-178 have well-known masses and radii. This means that planetary scientists can discover their densities, a clue about the composition of the planets (SN: 5/11/18).

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Both systems also offer another advantage: the planets are packed so close to their stars that most engage in a delicate orbital dance called a resonance chain. Each time an outer planet completes an orbit around its star, some of its nearest sister planets complete several orbits.

Resonance chains are fragile arrangements and taking a planet even slightly out of its orbit can destroy them. That means the TRAPPIST-1 and TOI-178 systems had to form slowly and smoothly, says astronomer Adrien Leleu of the University of Geneva.

The planets in TOI-178 are involved in a delicate orbital dance called the resonance chain that suggests the system formed smoothly. This video illustrates this rhythmic dance: as an outer planet completes a complete orbit, the inner planets complete several orbits. Each full and half orbit is assigned a musical note. When the planets align, the notes harmonize.

“We don’t believe there could be giant impacts or strong interactions where one planet has ejected another planet,” Leleu says. That smooth evolution offers astronomers a unique opportunity to use TRAPPIST-1 and TOI-178 as test benches for planetary theory.

In a couple of articles, two teams describe these systems in unprecedented detail. Both defend the trend that astronomers expected from theories of how planetary systems are formed.

In the TOI-178 system, the densities of the planets are confusing, Leleu and colleagues report on Jan. 25 in Astronomy & Astrophysics.

"In the most vanilla scenario, we expect planets farthest from the star … would have larger hydrogen and helium components than the nearest planets," says astrophysicist Leslie Rogers of the University of Chicago, who did not participate in either study. . The closer to the star, the denser a planet should be. This is because the more distant planets probably formed where it is cold and there was more low density material like frozen water, instead of rock. In addition, starlight can remove atmospheres from nearby planets more easily than distant ones, leaving inner planets thinner or no-atmosphere atmospheres (SN: 01/07/20).

The TOI-178 completely dispels that trend. The innermost planets appear to be rocky, with densities similar to those of Earth. The third is "very spongy," says Leleu, with a density like that of Jupiter, but on a much smaller planet. The next planet has a density like that of Neptune, about a third of the density of the Earth. Then there’s one with about 60 percent of the Earth’s density, still spongy enough to float if you could put it in a water bath, and the final planet is similar to Jupiter.

“The orbits seem to point out that there has been no strong evolution of the formation (of the system),” Leleu says. "But the compositions aren't what we'd expect from a smooth lineup on the record."

The planetary septet of TRAPPIST-1, on the other hand, has a mysterious self-resemblance. Each world is about the same size as Earth, between 0.76 and 1.13 times the radius of the Earth, astrophysicist Eric Agol of the University of Washington in Seattle and colleagues reported in 2017 (SN: 22/02/17). In addition, at least three of them appear to be in the habitable zone of the star, the region where temperatures may be adequate for liquid water.

Now, Agol, Dorn and their colleagues have made the most accurate measurements of the TRAPPIST-1 masses yet. The seven worlds are almost identical to each other but slightly less dense than Earth, the team reported in the February issue of Planetary Science Journal. This means that the planets could be rocky although they would have a smaller proportion of heavy elements such as iron compared to Earth. Or it could mean they have more oxygen bound to the iron in their rocks, “basically rusting,” Agol says.

Three planets with different compositionsThe seven planets of TRAPPIST-1 appear to have similar compositions to each other, but different from Earth. They could have an Earth-like composition but with a smaller iron-rich core (center) or no core (left). They could also have deep oceans (right), but the three inner planets are probably too hot to last that long.JPL-Caltech / NASA

Three planets with different compositionsThe seven planets of TRAPPIST-1 appear to have similar compositions to each other, but different from Earth. They could have an Earth-like composition but with a smaller iron-rich core (center) or no core (left). They could also have deep oceans (right), but the three inner planets are probably too hot to last that long.JPL-Caltech / NASA

Rusty iron would not form a planetary core, which could be bad news for life, Rogers says. No nucleus can mean any magnetic field to protect the planets from the star’s flares (SN: 3/5/18).

However, it is not clear how to form heartless planets. “There are proposals on how to form those planets, but we don’t really have a candidate in the solar system where we see this,” Dorn says. Analogs of the solar system are bodies the size of asteroids much less massive than Earth.

Astronomers will soon be able to have better control over the compositions of the TRAPPIST-1 planets. The James Webb Space Telescope, which will be launched in October, will probe the planets ’atmospheres (if they have any) for signs of chemical elements that would reveal in more detail that they are made.

Rogers says the similarities of the TRAPPIST-1 planets to each other are not as striking as the differences between the TOI-178 planets. But they are still unexpected. If all the planets have identical compositions, then any model of formation has to explain it, she says.

While these systems challenge astronomers ’views on what kind of planets are possible, Dorn says, more multi-planetary systems will have to be discovered to say how strange they really are.

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