In Search of Alien Planets

Erik Petigura marvels at how far his field of astronomy has come since his childhood in the early 1990s, when he first became hooked on science after watching Carl Sagan’s Cosmos series.

“Back then we didn’t know of a single planet outside our solar system,” says Petigura, a postdoctoral scholar in planetary science and a Hubble Fellow at Caltech. “Now we know of 5,000.”

Today, Petigura spends his time studying planets orbiting other stars, known as extrasolar planets or exoplanets. “I’m interested in the story of how planets form, how our Earth formed, how the conditions for life arose, whether or not those conditions are common throughout the galaxy, and how planets interact with one another,” Petigura says.

An Uncommon Solar System

For Petigura, one of the most profound insights from exoplanet research in recent years is that our solar system is not a typical outcome of planet formation.

“For hundreds of years, we had a very nice, tidy story about the formation of the solar system,” he says. “We had an explanation for why the small, rocky planets are close in and the gas giants are farther out, and why the orbits of the planets are confined to a single plane.”

But that view has changed over the past five years. “We’ve learned that most systems have planets between the size of Earth and Neptune located closer to their parent stars than Mercury is to our sun,” Petigura explains. “In our solar system, we have no such planet. Why? Did some unusual event occur during the formation of the solar system? If so, what are the implications for the emergence of life on Earth and around other stars?”

Solving that mystery will require lots of data about different kinds of planetary systems. Toward that end, Petigura is working with Caltech astronomer Andrew Howard and a team of other scientists and engineers to design a next-generation spectrometer for the W. M. Keck Observatory. The Keck Planet Finder (KPF) will measure the masses and orbits of exoplanets with almost 10 times greater precision than its predecessor at Keck. The design of KPF is nearly complete, and it will be built and see “first light” in 2020 if the remaining funds can be raised.

“We will really benefit from having an upgraded instrument,” Petigura says. “We strive to increase the precision and efficiency of our instruments so that we can look at more planets, smaller planets, different types of planets.”

“In our solar system, we have no such planet. Why? Did some unusual event occur during the formation of the solar system? If so, what are the implications for the emergence of life on Earth and around other stars?”
- Erik Petigura

Astronomy Paradise

For optical astronomers such as Petigura, who investigate the heavens using visible light, Caltech is a paradise. “It’s one of the best places in the world to do astronomy,” he says.

One reason is Caltech’s access to instruments at Palomar Observatory in California and the W. M. Keck Observatory in Hawaii, which are home to some of the largest optical telescopes in the world.

But even more important for Petigura, Caltech is one of only a few academic institutions where postdocs can apply for time on giant telescopes as principal investigators. “That is not the case at other universities,” he says. “It’s the principal reason why I came here. I wanted to have the ability to come up with a proposal, get telescope time, and then execute my idea. Every semester, I’ve applied for and received telescope time, and it’s been transformational in my development as an independent scientist.”

Guardians of the Solar System

Moving forward, Petigura plans to spend some of his precious telescope time studying the connection between close-in rocky planets and distant gas-giant planets, especially as it relates to creating the conditions necessary for life. In our own solar system, for example, it’s thought that Jupiter’s gravitational pull sweeps away comets that otherwise might collide with Earth. Astronomers were treated to a dramatic example of this in 1994, when fragments of the disintegrating Comet Shoemaker-Levy 9 slammed into Jupiter.

“Comet Shoemaker-Levy 9 would have been about a thousand times more powerful than all of the nuclear bombs going off at the same time on planet Earth,” Petigura says. “Jupiter regularly sweeps things like that up. So maybe it’s not enough just to have an Earth-sized planet in an Earth-like orbit. You may also need to have a Jupiter-like planet in the same system.”

To learn how you can support the Keck Planet Finder or other exciting astronomy projects that help alien planet hunters like Petigura, please contact Ellen Jampol at 626-395-4374 or

Since this story was published, Petigura left Caltech in 2019 to join the UCLA faculty.

Giving Priorities