Our solar system is not as unusual as we thought

The Kepler-11 system, as imagined from above, is a “peas-in-a-pod” planetary system. Credit: NASA

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 Path near their stars, in many cases closer than Mercury is to the Sun.

Our Planetary system, in contrast, appears to be different.

“[Planetary scientists] have thought our Planetary 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 All other as in other systems, there is a large gap (the main Experience) between Mars and Jupiter, and the giant planets are also not packed very close together.

But a new study by Becker and a Club of researchers suggests that our Planetary system isn’t as unusual as previously thought. The study, published in The Astronomical Journal, is the Primary 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 Club examined both the sizes of exoplanets and their period ratios — the number of Intervals a given World takes to Path its Luminous sphere divided by the number of Intervals it takes the Upcoming-closest World to the Luminous sphere to Path — as a measure of how close the worlds are to All 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 Club 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 Path around their Luminous sphere in about 10, 13, 23, 32, 47, and 118 Intervals (period ratios of 1.3, 1.7, 1.4, 1.5, and 2.5, respectively). In our Planetary 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 Planetary system fits reasonably well into the Club’s peas-in-a-pod category.

Dividing line

Some peas-in-a-pod systems also have outer planets like our Planetary system. Deciding on a rule to designate inner and outer regions wasn’t Effortless, but the Club classified outer planets as those that take longer than 130 Intervals to Path their stars and are separated by period ratios greater than 5.

These dual conditions sorted some planets into inner regions, Regardless of 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 Intervals to Path their Luminous sphere (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-World systems that the Club categorized could also have more planets smaller than Jupiter, and hence eventually be reassigned into the peas-in-a-pod category.

The Club also created a separate category called “Toasty 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 Club 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 World Arrangement

The new classification scheme is also useful for understanding how planetary systems evolve. For example, the Club was able to distinguish peas-in-a-pod systems from Toasty Jupiter systems.

“This is an Crucial 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. “Perhaps Toasty Jupiters prevent that.”

Peas in a pod, with their similar sized planets, could also imply a Frequent physical process for how their planets grew from dust grains. In the process, such systems may have experienced collisions and Beyond growth.

“What is surprising about these systems is that even if they experience instabilities, they can Yet preserve their peas-in-a-pod arrangements as we have seen in simulations,” says Izidoro.

Dividing systems into groups also has the Edge 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 Path in less than two Intervals.

“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 Begin to build categories and try to understand what are the groupings of planets that exist,” says Becker. “We are really at an exciting time.”