When we look around the galaxy, do we see places that seem similar to our home? Or do we see completely different kinds of planets?
Is our Solar System special? Has it evolved in some unusual way that could mean habitable planets like Earth are rare?
Thirty years ago, our entire understanding of what planets could be like was limited to our own neighborhood: the Solar System. Now, as scientists have begun to discover planets around faraway stars, we can finally put ourselves in context.
This story sums up how the Solar System and its planets compare to what scientists have found elsewhere.
Kepler K2-138 Planetary System (Artist Concept) (2018-01-11) by NASA/JPL-CaltechNASA
What's Home Like?
There are many ways that the Solar System might be rare. For example:
We have eight planets. Is that a lot?
We orbit a single yellow star. Is that unusual?
SunNASA
Here Comes The Sun
Before we even consider all the planets, it's natural to wonder how rare the Sun is.
As it turns out, not that rare. But it's also not the most common kind of star.
Planets Under a Red Sun Artist Concept (2011-04-08) by NASA/JPL-CaltechNASA
Dimmer and Redder
Most of the stars in our galaxy are cooler than the Sun and have a reddish color. These are called "M type" stars.
Planets may form differently around M stars than they would around the Sun. And M stars emit a lot of radiation and energetic particles that could endanger life.
Lonely Or Not?
On top of that, most stars aren't by themselves — they come in pairs. In these "binary systems", the stars orbit each other.
Planets might go around one of the stars or both. It's a lot harder to find planets in binaries, so we've only discovered a handful so far.
Sun Emits a Solstice CME (2017-12-08)NASA
So in the grand scheme, the Sun is already far from the most common star.
But it may be that M stars and binaries are less likely to have planets. In that case, most planets would orbit stars more like the Sun. The Solar System wouldn't be unusual, at least in that way.
Illustration of TRAPPIST-1 Planets as of Feb. 2018 (2018-02-05) by NASA/JPL-CaltechNASA
What About Planets?
It turns out that two of the most common kinds of planets — which together make up more than half of all we've discovered — don't exist in the Solar System at all.
Earth Bigger, Older Cousin Artist Concept (2015-07-23) by NASA/Ames/JPL-CaltechNASA
Super Earths
One of the most common kinds of planets are a bit larger than the Earth, so they're called Super Earths.
But they're not necessarily like the Earth. Some could be barren rocks and others could be completely covered in water or ice.
It's not clear why we don't have a planet like this in our Solar System.
Webb Science Slide 9 by L. Hustak and J. Olmsted, STScINASA
Mini-Neptunes
The other common kind of planet we're missing is called a Mini-Neptune. These planets are probably icy, with thick atmospheres. Some may host oceans.
It's a little weird that even though we have two Neptune-like planets in our Solar System, there's nothing like a mini-Neptune.
Webb Science Slide 10 by Credit: NASA/JPL-CaltechNASA
Is Eight A Lot?
How about the number of planets in the Solar System — are we weird in that regard?
TRAPPIST-1 Planet Lineup (2017-02-22) by NASA/JPL-CaltechNASA
We haven't quite detected enough planets around other stars to be sure.
But it doesn't seem like it's so rare to have as many as eight planets around a single star.
In fact, stars slightly cooler than the Sun have about 3 planets, on average, with orbits less than 400 days long. That matches up pretty well with the Solar System, which has 3 that close, too (Earth, Venus, and Mercury).
Kepler-90 system (Artist's Concept) (2017-12-14) by NASA/Ames Research Center/Wendy StenzelNASA
What About Order?
If a star has multiple planets, it also seems like the bigger planets tend to be the ones farther away from the star. That matches up with our Solar System pretty well.
WASP-18b (Artist's Concept) (2017-11-29) by NASA/GSFCNASA
Then Again...
One major exception to this: "hot Jupiters". These giant planets are around Jupiter's size, but are so close to their star that they orbit in less than ~10 days.
To put that in perspective, Jupiter goes around the Sun every 12 years.
Around 1% of stars have hot Jupiters.
JupiterNASA
It's a lot more common to have a planet like Jupiter farther away from its star. Astronomers figure about 6% of stars like the Sun have the equivalent of our Jupiter — a planet comparably big in a similar orbit.
Exoplanet Orbits Youthful Star (Artist Concept) (2016-06-20) by NASA/JPL-CaltechNASA
It Gets Crazier...
Beyond hot Jupiters, there are even some planets (called "ultra short period") that orbit their stars faster than once per day. About half a percent of stars like the Sun host such a close-in planet.
There's nothing in our Solar System even a little bit like this.
Webb Explained Slide 12 by Credits: NASA and JPL/CaltechNASA
The Shape of Orbits
On the other hand, if a star hosts multiple planets, they tend to be in a "bullseye" pattern like in our own Solar System.
It's the planets that go around a star all by themselves that tend to follow more unstable, oblong paths.
Forking Paths
This could mean there's more than one common pathway that planetary systems evolve along.
Some stars might birth a lot of planets that all settle down in stable concentric circles, like the Solar System.
Planet With Four Stars Artist Concept (2015-03-04) by Image copyright: Karen Teramura, UH IfANASA
Others might go through transformative cataclysms. For instance, a Jupiter-sized planet might form far away from its star, but then migrate inward, scattering all the closer-in planets out into space. Then, any leftovers might show more oblong and misaligned orbits.
LIFE Photo Collection
What About Us?
That just leaves the final question: how rare is a planet like Earth?
Apollo 11 Mission image - View of moon limb,with Earth on the ho (1969-07-20)NASA
Somewhere in the Middle?
We haven't quite discovered enough planets — particularly small ones in far orbits — to be sure. But the current best estimate is that for every Sun-like star, there are 0.16-1.5 rocky planets that could potentially host liquid water on their surface.
Hot, Rocky World Artist Concept (2015-07-30) by NASA/JPL-CaltechNASA
In other words, at least 1 in 6 Sun-like stars could have that sort of planet. And at most, every star could have at least one.
But to be like Earth, those planets also have to have water and maintain an atmosphere. And even then...
Panoramic view of the Atlantic Forest in Saco Bravo, Paraty (2014-12-30) by Henrique Ferreira (CC BY 2.0)Museu do Amanhã
Who knows if life will be born?
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