By now you may accept heard the address that as abounding as 1/4 of all the sun-like stars in the Milky Way may accept Earth-like worlds. Briefly, astronomers advised 166 stars aural 80 ablaze years of Earth, and did a analysis of the planets they begin orbiting them. What they begin is that about 1.5% of the stars accept Jupiter-mass planets, 6% accept Neptune-mass ones, and about 12% accept planets from 3 – 10 times the Earth’s mass.
This sample isn’t complete, and they cannot ascertain planets abate than 3 times the Earth’s mass. But application some statistics, they can appraisal from the trend that as abounding as 25% of sun-like stars accept earth-mass planets orbiting them!
Now, there’s a actual important admonition here: these are planets that accept the aforementioned accumulation as Earth, but that doesn’t beggarly they are actual earth-like. The planets the aggregation could acquisition were actual aing to their ancestor stars, so they’d be actual hot, and uninhabitable. But the acceptable account is that if that trend in accumulation they saw is correct, the Milky Way is blowzy with planets the accumulation of the Earth! If some of them are in the accouter area of their star… well.
So a funny thing: I was cerebration about this actual botheration a brace of canicule ago, but from a altered angle. How abounding accouter planets are there in the Milky Way? Not aloof earth-mass, but additionally orbiting their brilliant in the alleged Goldilocks Zone, area temperatures are appropriate for aqueous water?
There’s a way to appraisal it. And it involves the planet afresh announced, Gliese 581g. This planet is about 3 times the Earth’s mass, and it orbits its brilliant in the appropriate place. We don’t apperceive what it’s fabricated of, if it has an atmosphere, or absolutely actual abundant about it at all! But accustomed its accumulation and temperature, it’s potentially habitable.
The ambit to the Gliese 581 arrangement is what gets me excited: it’s 20 ablaze years away. That’s close, compared to the all-inclusive admeasurement of our galaxy. So let’s accept Gliese 581g is the aing potentially accouter planet to us. Accustomed that assumption, we can appraisal the cardinal of potentially accouter planets in the absolute Milky Way! And the math’s not alike that hard.
The not-so-hard math
Extrapolating from our one example, let’s say that accouter planets are almost 20 ablaze years afar in the galaxy (as we’ll see, that cardinal can be a lot bigger or smaller, and the end aftereffect is still cool). That agency there’s one brilliant per cube 20 ablaze years on a side:
In the drawing, anniversary box is centered on a star, and the two stars are 20 ablaze years apart. That agency the cubes are 20 ablaze years on a side, right? If we accept stars with adequate planets are broadcast throughout the galaxy like this, again there is one brilliant per 20 x 20 x 20 = 8000 cubic ablaze years. That’s the body of accouter planets in the galaxy.
So how abounding cubic ablaze years are there in the galaxy?
A lot. Let’s say the Milky Way is a chubby 100,000 ablaze years across, and 2500 ablaze years thick. The blueprint of aggregate of a is
volume = π x ambit of disk2 x acme of deejay
volume = π x 50,0002 x 2500 = 2 x 1013 cubic ablaze years
Holy wow! That’s 20 abundance cubic ablaze years!
Now we aloof bisect the aggregate of the galaxy by the body of stars with planets to get
2 x 1013 / 8000 = 2,500,000,000 planets
Oh my. Yeah, let that bore in for a second. That’s 2.5 billion planets that are potentially habitable!
What does this mean?
Well, that’s a accomplished lot of planets! That’s what it means.
What’s cool, too, is that this cardinal isn’t all that far off from what you can appraisal application the address from yesterday. Something like 25% of the stars in the galaxy are like the Sun (that’s a asperous estimate, but aing enough). That’s 50 billion stars. If 25% of those accept earth-mass planets, that’s about 13 billion total, about bristles times the cardinal I got. I’d alarm that appealing close! We fabricated a lot of guesses here, so alike a agency of ten isn’t so bad. And we’re not absolutely comparing apples to apples, either, back they were attractive for earth-mass planets, and I was attractive for earth-like planets.
So anticipate about it: 2.5 billion accouter planets is almost abundant for every man, woman, and adolescent on Earth to anniversary accept a planet. You can see why I’m not too anxious with the exact math. Alike if my numbers are way off, there could be as few as hundreds of millions of planets, or as abounding as maybe hundreds of billions in our galaxy abandoned that we could alive on!
Again, the point actuality that mathematically speaking, there may be a lot of accouter planets out there. And who knows; some may be hardly accouter and we can terraform them. And again there are moons of worlds, too… I don’t anticipate I’m speaking too far out of academy if I were to brainstorm that for every absolute Terra Nova out there, there ability be three or four added planets we could alive on with some work.
Of course, I’m blank how we’d get there! But that’s an engineering problem, and accustomed abundant time — oh, say, a aeon or two — I brainstorm we can affected a lot of those issues.
If, and when, we do, there will be a lot of absolute acreage out there to blow about in.
Per ardua, ad astra!
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