The Disks That Make Solar Systems Flat | Eberly College Of Science

While rotating tiny planets at the end of their wires around a bright orange sun, some might wonder about the accuracy of these toy models of our solar system. Do the planets really align in a plane, or do their orbits crisscross around the sun at different angles? It turns out that the toy isn’t too far off, at least for this aspect (just don’t ask about the planets’ relative sizes or distance from the sun!).

Our solar system is actually pretty flat, with most of its planets orbiting within three degrees of the plane of the Earth’s orbit around the sun, called the ecliptic. This flatness extends to the asteroid belt between Mars and Jupiter, though some members of the region of icy objects past Neptune called the Kuiper belt are more extreme, with inclinations up to 30 degrees. Of course, one Kuiper resident, the demoted planet Pluto, is perhaps omitted in modern toys.

This relative flatness, which it turns out is not an unusual feature of solar systems, results from how stars and planetary systems form. This process begins with a slowly rotating, roughly spherical cloud of gas and dust, about one light year across. Eventually, a portion of this material collapses toward the center, forming a star, and the spinning cloud begins to flatten into a disk due to the rotation. It’s out of this rotating protoplanetary disk of gas and dust that planets are born, resulting in a relatively flat solar system. Eventually, when most of the gas has settled onto the star or planets or has dissipated, the system is left with a debris disk of planetary leftovers, much like our own asteroid belt or Kuiper belt.

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Some astronomers at Penn State study protoplanetary and debris disks to get a better idea of how planetary systems form. But not all stars and planets form in exactly the same manner—and not all planetary systems are flat.

“It’s an exciting time, because so many planets have been discovered in other solar systems, for example by NASA’s Kepler space telescope and Transiting Exoplanet Survey Satellite (TESS), and a lot of them look very different from the planets in our solar system,” said Rebekah Dawson, Shaffer Career Development Professor in Science and assistant professor of astronomy and astrophysics. “So, we have to come up with new ways of thinking about planet formation that can account for the diversity of planets we now know about.”

In addition to studying disks, researchers like Dawson study the exceptions to the norm, unusual stars and planets that could support or make us rethink current theories of formation. Together, these investigations help us improve our theories about how and where different kinds of stars and planets form, which could also help us determine what makes a planet habitable.