Check out the architecture of our solar system.
Little planets close to the star, big planets far away.
Inner worlds rocky, outer ones gaseous.
Scientists once supposed other star systems in the Milky Way would look about the same.
“That’s what we used to think,” says Andrea Ghez, UCLA professor of physics and astronomy.
But researchers reconsidered after finding the first few hundred or so exoplanets. “None of them conformed,” says Ghez.
“It was a rude awakening.”
Credit: ESO / M. Kornmesser
Surprise one: the discovery of planetary exotica, like HD 189733b—a gas giant (the size of Jupiter), yet unthinkably hot (2,000 degrees Fahrenheit), and crazy close to its parent star (only 2.2 days to complete an orbit).
“Jupiter-size planets, really close to their sun,” Ghez says. “That was very unexpected.”
Now we know there are lots of places like HD 189733b. Perhaps a few billion.
Surprise two: the size of other solar systems.
Compared to them, ours is huge. The distance between the Sun and Neptune, the outermost planet, is nearly three billion miles.
Elsewhere, compact systems are common. The seven worlds around TRAPPIST-1—“packed tightly, one after the other,” says Ghez—occupy a space of only six million miles.
That’s one-sixth the distance between the Sun and Mercury, the innermost planet.
Credit: NASA / JPL – Caltech
Surprise three: the odd orbits of other worlds.
In our solar system, planets go “roughly in a circle,” says Ghez. (Indeed, Earth’s orbit is a near-perfect circle.)
But about half the exoplanets have distinctly oval-shaped orbits; some are wildly eccentric.
HD 20782b practically dive-bombs its star—coming within a few million miles of the host, then looping out to an orbit over 200 million miles away.
“Perplexing,” says Ghez.
Surprise four: our Sun—a yellow dwarf, middle-aged, relatively hot—is in the minority. Less than 10 percent of stars in the Milky Way fit that description.
Instead, most are red dwarfs—older, colder, and smaller.
“The Sun is not a typical star,” Ghez says.
“It formed alone, most likely. Which is pretty unusual.”
Other stars have companions. “We believe most stars in our galaxy are born in pairs,” she says. “Some have a multiplicity of companions.”
Credit: ESA / NASA
Hardly an analog to the rest of the galaxy, the solar system seems an anomaly.
“Is that somehow an important piece as to why our solar system can support life? The jury’s still out on that,” says Ghez.
What is clear: our early inferences and preconceived notions about what’s out there, based on a sample of one, were wrong.
A single sample often is.
“So much of what I taught when I first started is inconsistent with what we know today,” she says.
But astronomers are intrigued, not intimidated, by surprise.
“That’s when life gets really fun,” says Ghez. “I feel like a kid in a candy shop.”
Credit: JPL-Caltech / NASA / R. Hurt
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