First of its kind study led astrophysicists to rewrite the story of how planets form

Author: Deanna Csomo Ferrell

A team of astrophysicists led by Lauren Weiss, professor in the Department of Physics and Astronomy at the University of Notre Dame, created the first-ever catalog of small, earth-like planets with Jupiter-like siblings (planets that share the same star) — a critical component in the search for life elsewhere in our universe. 

Tw 1200x675 Laurenweiss

The study in the Astrophysical Journal Supplement Series, the Kepler Giant Planet Search took a decade to complete.

“This catalog is the first of its kind and an unprecedented opportunity to explore the diversity of planetary systems that are out there with things that are like the solar system, but not exactly the solar system, and it gives us a chance to rewrite the story of how the planets form,” Weiss said. “The science question that I’ve been trying to answer over the past decade is: Of the other small planets like Earth that are out there, which of them have Jupiter siblings? Because this might be an important characteristic to look for, if we want to figure out where to find life.”

Previous research over the past several years has singled out Jupiter as one of the reasons for life on earth. During the formation of the solar system, Jupiter slingshotted rocky and icy debris and embryonic planets toward Earth’s current location. Jupiter still hurls debris in Earth’s direction today. The debris may have carried water to our planet intact, creating the oceans and later, fostering life.

Based on data collected from the W.M. Keck Observatory on Mauna Kea in Waimea, Hawaii, Weiss and collaborators recorded almost 3,000 radial velocities of 63 stars like our sun that host 157 known, small planets. The 157 small planets range from the size of Mars to the size of Neptune, and some of them have rocky surfaces that might be suitable for life. During the study, the team discovered 13 Jupiter-like planets, eight planets closer to the size of Neptune, and three companion stars.

Perhaps counterintuitively, large, gas-filled giant planets outside of our solar system are difficult to find because some common detection methods don’t work. The Kepler space telescope, which retired after nine years in 2018 after it ran out of fuel, had been an excellent tool for scientists to find small exoplanets that orbited close to their stars. It used the transit method, which measures tiny dips in the brightness of the companion star to indicate the presence of a planet as it orbits its star.

Gas giants, however, are usually much farther from their stars, and don’t cross in front of them with any practical regularity for astronomers. Jupiter, for instance, takes 12 years to orbit the sun. Also, unlike planets close to their stars, distant planets often have slightly tilted orbits as seen from earth, making the dips in brightness less prominent.

Weiss and collaborators used the radial velocity method, which uses Doppler spectroscopy. The team measured the “wobble” of a star as the waves appear to pull slightly closer and away from earth based on the gravitational tug from a large, orbiting planet.

“Jupiters are large and they pull a lot on the stars we can measure. We can find them if we take many, many measurements over time, which is exactly what I had to do,” Weiss said. For every star in the sample, she and collaborators observed the Doppler shift of the star’s light waves for a minimum of 10 nights and in some cases up to hundreds of nights.

“It varies depending on the star,” she said, adding that “observing” the stars wasn’t done by directly looking through the telescope. Astronomers control the Keck telescope from remote observing stations worldwide, including at Notre Dame.

Papers such as this are exciting because they require patience, forethought and belief in the scientific spirit in order to complete, said Howard Isaacson, a research scientist in astronomy at the University of California, Berkeley. Scientists beginning long projects don’t know if they will ever be finished, and the projects may be multi-generational—several people join in and leave the project over time.

“Even if we participate for the entire project, we have to work on other projects in the meantime, while these slowly building projects simmer in the background gathering meaning,” said Isaacson, who was one of the project co-leaders. “And eventually the time is right, and we can say something profound about the universe, like how many solar systems out there have both small planets that may have rocky, Earth-like surfaces, and how many have massive Jupiter-like planets that command their own planetary systems.

“In the process we are putting our own place in the Galaxy into perspective.”

The sheer size of the and duration of collection of the dataset will allow other astronomers to answer important questions about the relationship between close-in planets and giants that are distant from their stars, said Erik Petigura, assistant professor of astronomy and astrophysics at the University of California-Los Angeles. He was part of the observation team and also designed the data management and visualization system that allowed the team to make sense of the large amount of data.

Currently, finding Jupiter-like plants is rare, only occurring around less than 10 percent of stars, according to Petigura.

“To answer questions about Jovian planets you need to observe stars for a decade or longer,” he said. “These datasets do not come into existence by accident, and require the audacity and long-term vision to see them through to completion.

“This is a testament to Prof. Weiss’s leadership and vision.”

Though Weiss was excited about the discovery of the Jupiter-like planets, the catalog of earth-and-Jupiter-like planetary systems is the aspect that will help astronomers in years to come. This paper, for instance, is the primary paper in the Kepler Giant Planet Search for which future papers will be based. Some will describe architectural patterns observed in planetary systems, the efficiency of detection of planets, and the joint occurrence of giant and small transiting planets.

“Probably the thing I'm most excited about is revisiting this story of how the Earth formed,” Weiss said. “Now that we have more information about what other kinds of planetary systems are out there, we’re looking for patterns, finding new discoveries, and these possibilities really excite me.”

In addition to Weiss, other collaborators on the study include astronomers from the University of California, Berkeley; the University of Southern Queensland, Australia; California Institute of Technology, Pasadena, California; IPAC-NASA Exoplanet Science Institute, Pasadena, California; University of California-Los Angeles, California; University of Chicago, Chicago, Illinois; University of the Pacific, Stockton, California; University of Nevada, Las Vegas, Nevada; Nevada Center for Astrophysics, Las Vegas, Nevada; The Pennsylvania State University, University Park, Pennsylvania; University of California, Irvine, California; University of Hawaii, Honolulu, Hawaii; Princeton University, Princeton, New Jersey; University of California-Riverside, California; University of California-Santa Cruz, California; Gemini Observatory/National Science Foundation’s NOIRLab, Hilo, Hawaii; and the University of Kansas, Lawrence, Kansas.