Scientists discover a hidden pattern in planet orbits

A planet’s orbit defines its movement around a star. Earth follows a nearly circular orbit, but exoplanets often take different paths. Some have very elongated orbits, while others resemble Earth’s circular pattern.

UCLA astrophysicists have now measured the orbits of exoplanets, from Jupiter-sized to Mars-sized, and found an interesting trend.

Smaller planets tend to have nearly circular orbits, while larger planets follow more elliptical paths. This suggests that large and small planets may form in different ways.

Bigger planets with different orbits

“What we found is that right around the size of Neptune, planets go from being almost always on circular orbits to very often having elliptical orbits,” said UCLA postdoctoral researcher Gregory Gilbert, who led the study.

The research team analyzed data from NASA’s Kepler telescope. This telescope observed 150,000 stars and detected exoplanets by measuring dips in brightness as they passed in front of their stars.

These brightness patterns, called light curves, revealed new details about planetary orbits.

The challenge of measuring planet orbits

One of the biggest hurdles was ensuring accurate modeling for all 1,600 light curves.

“If stars behaved like boring light bulbs, this project would have been 10 times easier,” said Erik Petigura, a UCLA professor of physics and astronomy.

“But the fact is that each star and its collection of planets has its own individual quirks, and it was only after we got eyes on each one of these light curves that we trusted our results.”

This is where UCLA undergraduate Paige Entrican played a crucial role. She developed a specialized visualization tool and manually inspected every light curve.

“Reviewing the data was a meticulous process that required careful inspection of all data products to ensure the validity of our results,” said Entrican.

“Several times during this project, I identified failure modes that only affected 1% of all our stars. But we needed to update our analysis to be robust to these issues and go back and reprocess the entire data set.”

Formation processes of planets

The divide between circular and elliptical orbits aligns with other known patterns in exoplanet studies. Gilbert noted that small planets are more common than giant planets.

“Large planets need metal-rich stars in order to form; small planets do not. Small planets have low eccentricities, and large planets have large eccentricities,” said Gilbert.

The connection between planet size, orbit shape, and star composition suggests two different formation processes for planets.

How planets take shape

Scientists believe planets start as small space rocks that collide and merge over time.

If a growing planet reaches a certain size, it can capture large amounts of hydrogen and helium, turning into a gas giant like Jupiter or Saturn. However, planets larger than Neptune are rare.

The growth of a giant planet requires a process called runaway accretion, where gas accumulation accelerates dramatically. This can only happen around stars with high levels of elements heavier than helium.

Planets with elliptical orbits

Larger planets with elliptical orbits indicate a more chaotic formation history. These planets likely experienced strong gravitational interactions that disrupted their paths.

Eccentric giant planets may have also influenced their neighbors, triggering major impacts like the one that formed Earth’s moon. In exoplanet systems, such collisions could involve planets much larger than Earth.

A tribute to Kepler

“It’s remarkable what we’ve been able to learn about the orbits of planets around other stars using the Kepler Space Telescope,” said Petigura.

“The telescope was named after Johannes Kepler, who, four centuries ago, was the first scientist to appreciate that the planets in our solar system move on slightly elliptical rather than circular orbits.

“His discovery was an important moment in human history because it showed that the sun, rather than the Earth, was at the center of the solar system. I’m sure Kepler, the man, would be delighted to learn that a telescope named in his honor measured the subtle shapes of orbits of Earth-size planets around other stars.”

This study marks a significant step in understanding how planets form and evolve. The differences in orbit shape between small and large exoplanets provide new clues about their origins.

With ongoing research and advanced telescopes, scientists will continue uncovering the mysteries of planetary formation.

The study is published in the journal Proceedings of the National Academy of Sciences.

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