E27 # The Weirdest Solar System We've Found So Far? You May Be In It

The Basics

Before we found the first exoplanets — planets orbiting other stars — it seemed reasonable to suppose that other planetary systems looked like ours: small, rocky planets close to a Sun-like star, a big Jupiter and a few other gas giants farther out.

But after a quarter century of discovery revealing thousands of exoplanets in our galaxy, things look very different. In a word, we are “weird” — at least among the planetary systems found so far.

Just how weird is still a matter of debate. And weirdness is relative. We’ve detected “hot Jupiters” in scorching, star-hugging orbits around their stars, where a “year” — one trip around the star — takes only a few days. We’ve found a string of small, rocky worlds, all in Earth’s size-range, in lock-step orbits around a tiny red-dwarf star called TRAPPIST-1. We’ve seen systems with one or more planets that are larger than Earth and smaller than Neptune. The properties of these worlds are a mystery because they’re unlike anything in our solar system — and yet, they’re among the most common types of exoplanets discovered so far.

In all this variety, we’ve seen nothing yet that quite resembles our own setup: a Sun-like star with a retinue of rocky planets close in and more distant gas giants (including a domineering Jupiter).

Planetary patchwork

We’re also notable for what we don’t have. Those planets larger than Earth and smaller than Neptune, for one. Systems like TRAPPIST-1 also have multiple planets in nearby orbits that are similar to each other in size and mass. For us, it’s a bit of a hodge-podge.

“Mercury and Mars are less massive than Venus and Earth,” says Yasuhiro Hasegawa, a NASA Jet Propulsion Laboratory researcher who studies the formation of planets and disks around stars. Why do we start with a small planet, Mercury, then have a relatively big Venus and Earth, then a smallish Mars?

And speaking of Mercury, why is our innermost planet so far from the Sun? You could fit the entire TRAPPIST-1 system of seven planets well within Mercury’s orbit. Many other systems detected so far also have planets in orbits far closer to their stars.

“Why is there no planet within Mercury’s orbit?” Hasegawa asks.

Nothing closer to the Sun than Mercury, a small Mars just beyond a bigger Venus and Earth, a really big Jupiter in a distant orbit. “That kind of configuration currently seems very rare,” Hasegawa says.

Jupiter's mood swings

Some of our strangeness, of course, is likely an artifact of our limited technology. Detecting systems like ours, with planets in years-long orbits around middle-weight, yellow stars, is far more difficult with present methods than finding planets in short orbits around small red dwarfs. Such planets are easier to detect by the “transit” method, when a telescope measures a tiny dip in starlight as a planet crosses the face of its star; planets in longer orbits require far more observation time to find them.

Credit: NASA