Welcome to the Galactic neighborhood

We have a lot of weird neighbors. By neighbors I mean planets orbiting other stars.  And by weird I mean that most of the extra-solar planets that we know look a lot different than the planets in the Solar System.

To start with, let’s take a look at our neighbors’ houses.  By houses I mean the stars that our planetary neighbors orbit around.  Our house — the Sun — is a 4.5 billion year old G star.  Here is what our neighbors are living in:

Distribution of the different types of stars near the Sun.  Credit: Franck Selsis via http://www.solstation.com/stars/pc10.htm.
Distribution of the different types of stars near the Sun. Credit: Franck Selsis via http://www.solstation.com/stars/pc10.htm.

This plot shows all the stars within a 30 light year radius of the Sun.  There are a whole lot more trailers (small, cool stars) than mansions (big, hot stars).  Of the more than 400 stars in this catalog, the Sun is in the top 8% in brightness. That’s a pretty high-class house we live in here in the Solar System!  Not the fanciest one around, but a lot swankier than average.

But you might not really want to live in the very fanciest houses (in this analogy, at least).  Those houses don’t stick around for very long.  Cool stars like M or K stars remain constant in brightness forever (seriously, for longer than the age of the Universe).  Hot stars like A stars brighten on much shorter timescales (hundreds of millions to a billion years), then burn up and turn into red giant stars in just a fraction of the time that life has existed on Earth.  “Trailer” stars definitely have some advantages in that regard.

Now let’s look at a lineup of our neighbors.  This plot shows planets (including some unconfirmed planet candidates) that we know.  Each dot shows the size (y axis) vs the orbital distance for an individual planet.  Earth is included for scale.  The color of each dot represents the kind of star it orbits: G stars are green, hotter (fancier) stars are yellow/red and cooler (trailer-esque) houses are blue/black.  These planets orbit stars that are a lot farther away than the 30 light year catalog above.


This is the part where we meet the neighbors and they are six-footed purple aliens.

Almost none of the planets that we know of resemble those in the Solar System.  The closest planet to the Sun is Mercury, at about 0.4 AU (1 AU = 1 Astronomincal Unit = the Earth-Sun distance).  Among our neighbors there are a ton of planets closer to their stars than Mercury!  Some of them are gigantic, as big as Jupiter.  Others are Earth-sized or a bit larger.

With current techniques we couldn’t detect the Solar System around another star.  So of course our neighbors don’t look like us!  Given 12 or more years of observations we could detect Jupiter.  There are indeed a handful of planets on Jupiter-like orbits that are now known.  But Earth and the other rocky planets are simply too small, and Saturn, Uranus and Neptune are too far out.

For now, lets’ make groups of some of the clumps of planets that we see.


Hot Jupiters are really big planets that are close to their stars.  Hot super-Earths are planets close to their stars and a little bigger than Earth (the super- is for the planets’ sizes not for the superpowers of their inhabitants, unfortunately — those planets are too hot for life).  These are not the only groupings of planets but they are the most obvious.

Over the next several weeks we’re going knocking on some of our neighbors’ doors.  We will meet a hot Jupiter, a system of hot super-Earths, a system with a gas giant on a very stretched-out orbit, and a bunch of other weirdos!  We’ll even meet some planets at the right temperature for life.  Of course, it’s a big leap from having the right temperature to having life.  We’ll jump into that can of worms this Spring when we discuss the habitable zone.

Comments and suggestions welcome.   Boom!

6 thoughts on “Welcome to the Galactic neighborhood

  1. Planets 2-3x the size of Earth have measured densities of 1-2 g/cc. This shows they have large amounts of gas. The term “super-Earth” is not appropriate, as they are not scaled-up Earths. The better term is mini-neptune or sub-neptune, to convey the likely gaseous content.

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