PLANETPLANET

Carnival mirrors.  They make you look…. different.  Here’s what my son Zack looks like with one carnival-mirror effect on my computer.

If every planet we saw was reflected in carnival mirrors, how would we figure out what the planets really look like?  It turns out this is how it actually works!  Every time we detect a planet there are distorting effects.  Astronomers “un-distort” their observations to figure out what the planet’s true properties are.

Over the last several weeks we’ve been meeting our Galactic neighbors.  We’ve run into some serious weirdos: hot Jupiters, super-Earths (and mini-Neptunes), planets on stretched-out orbits, and even homeless planets that don’t orbit a star. Now these planets are truly weird: the distortions have already been removed.  It’s starting to feel like we are the only normal ones around here!

Why aren’t we finding planetary systems that look like the Solar System?  Well, the most important question is what planets can we actually find?  We can only compare with what we can see.  If the carnival mirror isn’t in view of the right planets then we’re out of luck.

If every star hosted a planetary system like ours, what would we see?  After a solid 10+ years of observations we would detect Jupiter.  That’s it.  The rocky planets are too small to detect, and Saturn, Uranus and Neptune are too far away.  Now, plenty of planets like ours have been detected.  Earth-sized and smaller planets, Neptune-sized, Saturn-sized, Jupiter-sized ones.  But not laid out in systems like ours.  We can find small planets in “sweet spots” where they are easier to find.  These sweet spots are (for most techniques) located very close to the star, which is why there are so many different kinds of “hot” planets known.

We do know of a few planetary systems that sort of look like the Solar System.  Let’s check out three of them.

First up: HD 13931.  HD 13931 is a star about the same size and brightness as the Sun.  It has one known planet with a mass and orbit very close to Jupiter’s.  It is a good example of what you would see if you stared at the Solar System for 10+ years.  The most interesting thing in this system is what is missing: there are no gas giants on closer orbits in the HD 13931 system.  Close-in giant planets can prevent planets like Earth from forming.  The Jupiter-like planet has a nearly circular orbit, a good thing for forming planets like Earth.  Of course, we can not detect rocky planets around the star.  But from what we can see there are no show-stoppers.

Next up: OGLE-2006-BLG-109.  This system has two giant planets orbiting a star about half the size of the Sun.  Scaled to the brightness of the star, the planets have similar sizes and orbits to Jupiter and Saturn.  This makes the system seem kind of like a shrunk-down Solar System, although only Jupiter and Saturn have been found.  This is another system where what is most interesting is what we don’t see: the rocky planets.  The “Saturn” in the system has been tentatively measured to have a slightly stretched-out orbit.  Not great for forming rocky planets but nothing tragic.

Last up: Kepler-90.  This is basically a mini-Solar System!  It has seven known planets, all squished inside Earth’s orbit.  Here is the layout of their orbits:

The two outer planets are gas giants and the five inner planets are super-Earths or mini-Neptunes.  This is like the Solar System, where Jupiter and Saturn’s orbits are wider than the rocky planets’.  The two closest-in planets are 1.2 and 1.3 times larger than Earth and are probably rocky.  The next three planets are 2.5 to 3 times larger than Earth and are more likely to be mini-Neptunes.

The star (Kepler-90) is slightly brighter than the Sun, so all of the super-Earths and mini-Neptunes are too hot for life.  Still, the system as a whole is starting to look pretty familiar.  Kind of like the Solar System seen through a carnival mirror!

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This marks the end of the Welcome to the Galactic neighborhood series.  Except for a special announcement coming April 17th…

Next up: a series on the habitable zone co-authored by Franck Selsis.

Questions, comments, words of wisdom?