Astronomers have discovered more than 300 exoplanetary systems that have three or more known planets. Most of these planets are about the same size and spaced close together, earning them the moniker “peas in a pod.” They also orbit near their stars, in many cases closer than Mercury is to the Sun.
Our solar system, in contrast, appears to be different.
“[Planetary scientists] have thought our solar system is not standard in its spacings,” says astronomer Juliette Becker of the University of Wisconsin-Madison. In particular, the rocky planets are not as close to each other as in other systems, there is a large gap (the main belt) between Mars and Jupiter, and the giant planets are also not packed very close together.
But a new study by Becker and a team of researchers suggests that our solar system isn’t as unusual as previously thought. The study, published in The Astronomical Journal, is the first to use the entire catalog of exoplanets to examine the architecture of planetary systems.
Like peas in a pod
To sort planetary systems into groups, the team examined both the sizes of exoplanets and their period ratios — the number of days a given planet takes to orbit its star divided by the number of days it takes the next-closest planet to the star to orbit — as a measure of how close the worlds are to each other.
In total, about 80 percent of the systems with three or more planets fit into the peas-in-a-pod category.
“There is not a strict criterion for [whether] something is or isn’t peas in a pod,” says Becker.
The team considered systems to be peas in a pod even if their planets are not perfectly evenly spaced. For example, Kepler-11 has six known planets that complete one orbit around their star in about 10, 13, 23, 32, 47, and 118 days (period ratios of 1.3, 1.7, 1.4, 1.5, and 2.5, respectively). In our solar system, Mercury, Venus, Earth, and Mars have period ratios of about 2.6, 1.6, and 1.9, respectively.
Overall, the study showed that our solar system fits reasonably well into the team’s peas-in-a-pod category.
Dividing line
Some peas-in-a-pod systems also have outer planets like our solar system. Deciding on a rule to designate inner and outer regions wasn’t easy, but the team classified outer planets as those that take longer than 130 days to orbit their stars and are separated by period ratios greater than 5.
These dual conditions sorted some planets into inner regions, despite being separated by large gaps. The researchers called such systems gapped peas in a pod. Kepler-62 is an example, with five planets that take about 6, 12, 18, 122, and 267 days to orbit their star (period ratios of 2.2, 1.5, 6.7, and 2.2, respectively).
“It’s possible some of those gaps are planets we haven’t detected yet,” says planetary scientist André Izidoro of Rice University, who wasn’t involved in the study.
Some of the single- and two-planet systems that the team categorized could also have more planets smaller than Jupiter, and hence eventually be reassigned into the peas-in-a-pod category.
The team also created a separate category called “warm Jupiter” systems for those that have planets larger than Jupiter in the inner regions with smaller planets. The additional distinction was to account for the greater variability in these planets’ masses and period ratios.
Overall, the team was able to classify about 97 percent of planetary systems with three or more planets. The classification scheme is also broad enough to include smaller planets that could be discovered with more powerful telescopes in the future.
Architecture of planet formation
The new classification scheme is also useful for understanding how planetary systems evolve. For example, the team was able to distinguish peas-in-a-pod systems from warm Jupiter systems.
“This is an important step in trying to understand how these planetary systems get to these [peas in a pod] preferred architectures,” says planetary scientist Darryl Seligman of Michigan State University, who was not involved in the study. “Maybe warm Jupiters prevent that.”
Peas in a pod, with their similar sized planets, could also imply a common physical process for how their planets grew from dust grains. In the process, such systems may have experienced collisions and further growth.
“What is surprising about these systems is that even if they experience instabilities, they can still preserve their peas-in-a-pod arrangements as we have seen in simulations,” says Izidoro.
Dividing systems into groups also has the advantage of identifying outliers, which can entice researchers to think about unique process by which planets form and evolve.
For example, TRAPPIST-1 and Kepler-42 are miniature-sized peas-in-a-pod systems, the latter having planets that complete one orbit in less than two days.
“Breaking a classification scheme is sometimes what pushes science forward,” says Seligman.
Much effort has been put into classifying the nearly 6,000 discovered exoplanets to date by size and orbital period. But examining their systems as objects has been limited.
“It’s actually really difficult to get enough planets to start to build categories and try to understand what are the groupings of planets that exist,” says Becker. “We are really at an exciting time.”
