Biggest Questions in Science: Mike Brown

Depiction of the Ptolemaic system of the universe

Humans have consistently misjudged their surroundings

From time immemorial, people have sought to understand their place in the universe by making sense of how objects moved in the night sky. By A.D. 170, the astronomer and mathematician Ptolemy had honed the geocentric model, which held that the sun and the six planets visible to the naked eye—Mercury, Venus, Earth, Mars, Jupiter, and Saturn—revolved around the earth. That view held for more than 1,400 years.

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Science shapes a new view of the solar system

Through careful observations, theory and mathematical prediction, Nicolas Copernicus came to view the solar system as six planets circling the sun. As telescopes improved and scientists improved at mathematically predicting orbits, astronomers discovered Uranus in 1781, Neptune in 1846, and Pluto in 1930. This created a view of a solar system that contained nine planets, from Mercury to Pluto. For the rest of the 20th century, this view prevailed.

Credit: ANDRZEJ WOJCICKI/Science Photo Library/Getty Images

Mike Brown points out the Predicted Orbit of the 9th Planet at the Caltech Seismology Lab

Astronomers discover distant, icy bodies

Beginning in the 1990s, Mike Brown, an astronomer at Caltech who shared the 2012 Kavli Prize in Astrophysics with astronomers David Jewitt and Jane Luu, investigated objects in the Kuiper Belt, a region of icy comets and other small objects beyond Neptune’s orbit. He and other astronomers discovered a Pluto-sized object orbiting there, which they named Eris, after the Greek goddess of discord and strife.

Watch Mike Brown talk about the search for our solar system’s ninth planet. Credit: FREDERIC J BROWN/AFP/Getty Images

NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto

Pluto is demoted

Eris lived up to its name. Brown found that it was about as large as Pluto (shown here), and also had an elongated orbit beyond Neptune’s. Unlike the eight planets from Mercury to Neptune, neither Eris nor Pluto had gravity strong enough to clear its own neighborhood of other objects. In a decision that stirred controversy, the International Astronomical Union considered Brown’s findings, then demoted Pluto from planet to dwarf planet.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

“When we finished these big surveys and found these fascinating little worlds, one of the things that we also found is that there's nothing else large out there. … In some ways, I was sad about that. … And it took me a while to get over it and to keep on being excited about studying just these little worlds. And then the most amazing thing happened.”

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Clues to a Distant Planet In 2003 the view of the solar system began to change once more. Brown discovered a world called Sedna far outside the Kuiper Belt with a distant, elliptical orbit. Other astronomers discovered five other distant worlds with similar distant, elliptical orbits, and the six orbits lined up, as if pointing to something. Elliptical orbits form only when the gravity of a massive object pulls or kicks a smaller object out of a circular orbit. What had pulled or kicked these objects into such strange orbits? Credit: Youssef A. Khalil

The Case for Planet 9

Press play to explore the evidence for Planet 9 here in this 3D interactive model of the outer solar system, made with Mike Brown’s data.

Credit: Wojtek Urbanek

The Pan-STARRS1 Observatory on Halealakala, Maui

Searching the Sky for Planet 9

Computational models from Brown and his Caltech colleague Constantin Batygin predict that Planet 9 orbits 20 times farther away from the sun than Neptune does. Even a big planet that far away would be faint to see and hard to spot. But the models place Planet 9 in the Taurus constellation, and the two Caltech astronomers, as well as a team led by Scott Sheppard of the Carnegie Institution for Science, are using Hawaii’s Pan-STARRS telescope (shown) to search for it.

Credit: Rob Ratkowski

Artist's on the surface of the exoplanet TRAPPIST-1f, located in the TRAPPIST-1 system in the constellation Aquarius.

Is our Solar System Unique?

Beyond the hunt for Planet 9, astronomers are looking to other stars for insights into our solar system. They have spotted 2,500 stars with planetary systems, but none with a planetary system like our own. NASA's two-year Transiting Exoplanet Survey Satellite (TESS) mission, launched in 2018, has surveyed thousands of stars and found hundreds of exoplanets, and in January found its first Earth-sized exoplanet that may harbor liquid water. This mission could also yield clues to how our solar system took shape and how rare life is in the universe.

Credit: NASA/JPL-Caltech

A brightly reflective Enceladus appears before Saturn's rings, while the planet's larger moon Titan looms in the distance

New Homes for Alien Life

Whether life exists elsewhere in our solar system, much less our galaxy, is another question that scientists are addressing. Our solar system’s ocean moons—Saturn’s Enceladus (shown) and Jupiter’s Europa—may harbor deep-sea hydrothermal vents like those on Earth, which teem with microbes and exotic lifeforms. NASA’s Oceans Across Space and Time program, led by Georgia Tech’s Britney Schmidt, is developing a small, long-range, deep-water, under-ice robotic submarine that could one day be put on a rocket to Europa to look for life in its chilly ocean.

Credit: NASA/JPL-Caltech/Space Science Institute

Mike Brown, Professor of Planetary Astronomy

“Humans explore. The first humans, I'm sure, looked across the plains and wanted to know what was on the other side. Humans crossed oceans to find what's on the other side of oceans. The solar system is the biggest neighborhood that we have. The exploration that's happening now is making our biggest neighborhood a little bit bigger.”

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