“I have departed from this planet and I have left behind my poor earthly ones with their occupations which are as many as they are useless; at last I am living in the scintillating splendor of the stars, each of which used to seem to me as large as millions of suns.” -Jules Massenet
Each star in the night sky, just a distant, shining point of light, holds the same potential as our Sun. The potential for other worlds, for life, for civilization both as we know it and as we can only imagine it. On a clear, dark night, the possibilities seem almost limitless.
If you had asked me, 20 years ago, about the possibilities, I would have been able to tell you a wonderful story about what other worlds there could be around other stars. But it isn’t the early 1990s anymore; we’ve detected planets orbiting other stars!
You likely think of our Sun as being fixed in space, with the planets orbiting around it. To an excellent approximation, this is true; after all, the Sun has about 99.8% of the mass in our Solar System! But in reality, just as the Sun’s gravity pulls the other planets towards it, the other planets gravitationally pull on the Sun. If Jupiter were much larger than it is, our Solar System would look, instead, like the animation above.
Now — this is really important — if you were very far away from the Sun and it were orbiting with another massive body, what would you see?
Well, if you were edge-on (as opposed to face-on) to that system, you’d see the Sun moving towards you for part of its motion, and moving away from you for the other part. As far as light is concerned, when the star is moving towards you, the light gets blue-shifted, while when it moves away from you, it gets red-shifted.
Watch this red-blue-red-blue oscillating pattern over enough time, and you can measure the
- masses of any orbiting planets, by the magnitude of the shift, and
- the distance from each planet to the star, by the period of the shift (and knowing Kepler’s Laws).
Unsurprisingly, the first planets ever discovered by this method were very massive and very close to their parent stars, because that’s the easiest kind to see using this method! But we have a second method…
Planetary transit! Normally, when you watch a star, you get a roughly constant amount of light coming from it. But if something passes in front of that star, it can block a certain percentage of the light. If your measurements are sensitive enough, you can detect even tiny changes in the amount of light emitted.
And again, if your alignment is perfect enough, planets in other Solar Systems will pass in front of their parent star, blocking the light, and allowing you to learn:
- the radius of the planet, by measuring the amount of light blocked, and
- the distance from the planet to the star, by measuring its period.
We used to talk about planetary transits only for Mercury and Venus, because they’re the only ones — as seen from Earth — that can block our Sun. (And it is spectacular!)
But this is how the Kepler Mission is currently finding planets, and all told, we’ve got over 1,500 planets we’ve now discovered outside of our Solar System!
And we’ve discovered some interesting things. Not just small, rocky planets like our four inner ones: Mercury, Venus, Earth and Mars.
(Although, we have discovered ones like this.)
And not just large, gas giants, like Jupiter, Saturn, Uranus and Neptune, although we have discovered planets like those.
We also detect planets that are “in-between.” In other words, they appear to be much more massive than the rocky planets we have, but they don’t appear to be as large as the gas giants we have.
What to make of these so-called super-Earths?
Well, if we really wanted to know what was going on, we’d be able to measure a planet by both methods: wobble and transit. If that were the case, we’d know its mass and it’s radius, and be able to say something definitive about that planet’s density, and perhaps, if we knew enough, what it was made out of!
Take a look at 55 Cancri, or rho cancri, a magnitude 6 star (just barely visible to the naked eye under ideal dark conditions) in the constellation of Cancer the Crab. (Its location, above, was compiled using the free-and-awesome-software stellarium.)
At only 40 light-years away, it is the brightest star in the night sky known to have planets orbiting it. First detected via the wobble method, we’ve been able to deduce at least four five planets orbiting it.
And while the outer four planets are gas giants like Jupiter, the innermost one is one of these super-Earths.
But what’s really remarkable? We’ve just measured — for the first time — transit and wobble due to this innermost planet! The transit is brand-spankin’ new, and marks the first time we’ve ever measured mass (via wobble) and size (via transit) for the same extra-solar planet!
So, what can we learn about it?
It’s really dense! At 10.9 g/cm3, it’s nearly double the density of Earth, the densest planet in our Solar System. It weighs in at more than eight times the mass of Earth, but is only 63% larger in radius than we are! How did it get like that?
It turns out that — rather than being something like a ball of metal — this is likely the dense, compressed core of a gas giant like Jupiter! Take a look at a cutaway of Jupiter to see what I’m talking about.
Rather than gas all the way down, Jupiter has a solid core, but a gaseous atmosphere. But if you brought Jupiter too close to the Sun, it would wind up like Mercury: stripped of all its atmosphere by the incredible pressure of the Sun!
But unlike Mercury, what’s left behind — the dense, rocky core — would be far more massive and dense than any planet we have today. In this particular case, the core is probably compatible with this body having originally been a Saturn-sized planet. But with its atmosphere stripped away, all that’s left is a small core barely larger than Earth, but far more massive and dense!
Thanks to Jon over at Universe Today for breaking this story, and I hope you all enjoyed learning about this new “first” for astronomy!