That NASA’s Kepler mission should discover some new planets isn’t really news at this point. The Kepler observatory is designed, after all, to do just that. At the end of last year it had discovered a total of 2,326 potential exoplanets – extrasolar planets. They’re potential because they require some pretty time-intensive confirmation.
Astronomers use Kepler to finely measure the total light output of tens of thousands of stars. The stars appear to dim briefly every once in a while, but only by a barely-perceptible amount. It’s not enough to cause them to twinkle (our atmosphere is responsible for the scintillation of the heavens, you know), but it is measurable, and often, it’s the same amount.
A likely possibility in the case of periodic dimmings and brightenings is that a planet is crossing in front of its star (from our perspective). This June, in fact, you’ll be able to observe a planetary transit right in our own neighborhood, as Venus passes between the Earth and the Sun. And you better try to catch it in June, too, because you won’t have another chance until 2117. So each dimming of a star is tallied as a potential planet. Then we just wait until the suspect passes in front of the star again, completing its orbit.
Most of the planets that Kepler has confirmed have short-period orbits: they don’t take very long to complete a revolution. The Kepler mission hasn’t yet been in operation for three years, so it’s unsurprising that it hasn’t had time to confirm longer-period planets. After all, in its brief lifetime so far, it wouldn’t have even had a chance to confirm Earth as a planet!
To confirm a planet, astronomers need to take note of a dip in brightness that could be explained by a transiting body. Then they wait until it does it again, and measure the time it takes. Then they wait the same amount of time to see if it does it again. If it passes those tests, you’ve got a planet. Naturally, it takes time to do it, as well as a considerable amount of effort to interpret the data. It isn’t nearly as simple a process as this summary would make it seem like. But, it’s NASA’s job to figure out how to do that. My job is only to marvel at how awesome the universe is.
The bit that caught my attention in this press release, however, is the bit about how astronomers are growing far more competent at quickly establishing the orbits of multi-planet systems. If we were to take our own solar system as an example, we’d find that only a handful of our planets, the interior rocky ones, are easy to study based on their orbits alone. Mercury revolves around the sun about four times while we complete one revolution; in the three years Kepler has been at work we could have built up a substantial file on it had we been observing it on an alien star.
Neptune, distant and patient, takes 165 Earth years to complete a revolution. It has been generally assumed that gas giants like Neptune and its cousins Saturn, Uranus, and Jupiter are the most prevalent planets in the galaxy. To date, we have discovered more gas giants than small rocky planets like Earth. However, gas giants have been easier to detect with less advanced methods. New data (including data from Kepler) is beginning to paint a different picture. More rocky planets are being discovered using some very complex techniques, and in fact, the rate of discovery exceeds that for gas giants.
Of course, that isn’t the whole picture, either. Even with Jupiter’s relatively modest year of eleven Earth years, it could be a while before anybody spotted it transiting the sun. Far out planets like Neptune might never be observed in our lifetimes. So when astronomers estimate that there are about 160 billion planets in our galaxy it isn’t because they’ve meticulously counted each one (though not for lack of trying). It’s a statistical estimate that, like many in the world of astronomy, is subject to revision as we obtain more data.
An artist's depiction of Kepler 16b, circling two suns. From NASA.
But wait, how do scientists know the mass and composition of a planet, anyway? You guessed it: complex measurements. By measuring the amount of light that a planet blocks, scientists can calculate its volume. The really tricky part is measuring the wobble of a star as the planet tugs on it back and forth. Just like the Moon’s gravity affects our tides here on Earth, our gravity affects the Sun (although only very minutely). By measuring the influence of a planet on its star, scientists can calculate its mass. Once you know its mass and volume, figuring out its density is the easy part. A small, massive planet is likely made of rock and metal, whereas a large, less dense planet is likely to be a gas giant.
So like I was saying, NASA’s scientists are getting much better at observing planets in a multi-planet system. Using a technique called Transit Timing Variation, astronomers use the differences in timing for each planet’s transit to calculate the gravitational effects of other potential planets; it allows them to quickly and accurately assess how many planets a given star system is hosting. If it sounds complicated, it is. Don’t let this video fool you; I bet it isn’t easy!
A billion other worlds
In case you thought I was going to let that comment about 160 billion planets slide, let me expand on it. Based on data we’ve observed so far, there seems to be about an average of 1.6 planets per star. A conservative estimate suggests that there are about 100 billion stars in the Milky Way, but there could be as many as half a trillion. Either way, that’s a lot of planets.
Very few of the planets that Kepler has confirmed so far seem as though they might be hospitable to life as we know it, but the data has only just begun to roll in. Even with just tens of confirmed planets under its belt, the Kepler mission has spotted some truly alien worlds. The planet Kepler 16b (pictured above, courtesy of NASA) circles two stars. The star KIC 12557548 has a planet so close it’s being boiled alive, and KOI 961′s planets’ orbits are only two days long. Other observatories are making similarly weird finds.
We know there are a lot of planets out there, even before we make any estimates. We know of over 750 right now, with more coming in regularly. So, yeah, 160 billion planets in the Milky Way (give or take). And we’re just one galaxy.
For ease of calculations, I just try to remember that there are about 100 billion stars in an average galaxy, and there are about 100 billion galaxies. When I say “about,” however, it must be understood that we really have no idea. Some galaxies contain just a couple tens of millions of stars, and others can contain trillions. Even the number of galaxies has been calculated based on one tiny patch of sky that we happen to be able to see clearly. Either way, when I say there are about ten sextillion stars in the known Universe, I don’t expect anybody to quibble.
Because ten sextillion looks like this: 10,000,000,000,000,000,000,000.
I wouldn’t even know how to handle it if you said I was off by a zero or two, because it’s all the same to me. It’s just a lot of friggin’ stars. You just cannot conceive of this number. Even if you gathered a trillion stars every day it’d still take you 27 million years to collect them all. The odds of you picking the Milky Way out of an enormous Universe-sized hat at random are…well, let’s just say you’re more likely to win the lottery 500 times than to find the Milky Way once.
The incredible vastness of the Universe is really too much to deal with coherently. There is so much out there beyond us, and we haven’t even begun to discover it all. The more I learn about the Universe, the more awestruck I become. That we can find planets so distant the starlight we’re measuring left before we were born is incredible to me.
An alien sunbeam hurtles for countless eons at blistering speeds in an awful, lonesome odyssey through space, a subtle field of nothing punctuated by subtler ripples of invisible forces, until just when it was going to fade into the background it chances to hit a telescope and it gives its life to our imaginations. That’s the wonder of the Universe, and I absolutely adore it.