ⓒ NASA/ESA and G. Bacon (STScI)
Physical Sciences

As It Turns Out, We Are Far From Alone in the Universe

How the Last 20 Years have Rewritten Our Understanding of the Universe

It seems logical enough: the humble eight planets swirling around our wonderfully stable star cannot possibly be the only planets in the universe. It’s a rather big place, afterall, and home to billions and billions of stars. And yet for the vast majority of human history, no one had ever seen or otherwise measured the existence of such a thing. Exoplanets—planets beyond our solar system—had to be out there somewhere, but where?

For hundreds of years, that question dogged scientists. But planets are awfully hard to see. Even behemoths the size of Jupiter or larger, are small compared to stars. Further complicating matters is that planets don’t emit light, they merely reflect the light of their host stars. That is if they aren’t obscured by it altogether. At best they’re a faint, dull glow in the inky blackness of space. It’s what many scientists have likened to trying to spot a moth flapping in front of a searchlight from miles and miles away. How do you find something like that?

FINDING THE MOTH

An artist’s conception of 51 Pegasi b. ⓒ ESO/M. Kornmesser/Nick Risinger (skysurvey.org)

In 1995 two teams of scientists working independently cracked the case wide open. A group in Italy and another working at the Lick Observatory in Santa Cruz, were finally able to verify the existence of a planet now called 51 Pegasi b. After decades of false promises, the hunt for exoplanets had its first bite.

Now, just 20 years later, scientists have confirmed the existence of almost 2,000 exoplanets and identified more than 5,000 candidates, most of whom are likely to be confirmed. (NASA’s Jet Propulsion Laboratory keeps a running tally here.) In fact, current estimates predict that there may be anywhere from 100 to 400 billion exoplanets in our galaxy alone. In the span of just two decades, the hunt for exoplanets has gone from theory to reality to virtual deluge.

our skies are now populated with a weird and wonderful host of planets unlike anything we have ever known—or known was possible.

Perhaps most exciting of all, much of the leg work of identifying candidate planets has been done not in high tech labs or around university tables, but by citizen scientists with little or no training. Sites like planethunters.org have made it possible for anyone to comb through the data collected by NASA’s Kepler space telescope looking for indications that a given star hosts at least one planet. That means that on any given night, you (yes you) can sit down at your computer and look for hidden worlds that are both long ago and far away.

THE GRAPHS AND THE GLORY

Though we were once literally in the dark about how to look for exoplanets, there are now a variety of ways to successfully identify our distant planetary neighbors.

Kepler and a host of other telescopes use what’s known as the transit method. Though planets are much, much smaller than stars, when they pass in front of a star, that star’s brightness appears to dim ever so slightly. That brief dimming produces telltale dips in graphed data that can be picked out by amatuer planet sleuths.

The wobble method—whereby planets are detected by measuring the very small wobbles that their gravitational pull causes in a host star—has also produced some stunning results.

This tiny glowing dot is the first exoplanet ever directly imaged by Hubble. It’s trillions of miles from Earth. ⓒ NASA,ESA, P. Kalas, J. Graham, E. Chiang, E. Kite (University of California, Berkeley), M. Clampin (NASAGoddard Space Flight Center), M. Fitzgerald (Lawrence Livermore National Laboratory), and K. Stapelfeldt and J. Krist (NASAJet Propulsion Laboratory).

Lest you think that the hunt for exoplanets is all graphs and very little glory, know that we can now capture their images as well. The world’s most famous space telescope, the Hubble, produced the first image of an exoplanet in 2008. That historic image was made possible by Hubble’s powerful optics, designed in part by National Science Medalist Dr. Sandra Faber. Other images are forthcoming from the Subaru Telescope at the Mauna Kea observatory in Hawaii, which another National Science Medalist, Dr. James Gunn, played a role in designing.

Though perhaps most famous for its images, Hubble has also unlocked some of the mysteries of these alien atmospheres—probably our best hope for detecting possible signs of life. Using a spectrometer to split light into its constituent colors and reveal the “fingerprints” of chemical molecules, Hubble has identified water vapor, methane, and other particles in exoplanet atmospheres. It’s also provided temperature maps of planets orbiting distant stars. Spitzer, another space telescope, has been able to further fill in the picture by providing scientists with a way to study weather patterns. Subaru too will provide information about exoplanet atmospheres.

A WHOLE NEW WORLD

Thanks to these incredible leaps of science, our skies are now populated with a weird and wonderful host of planets unlike anything we have ever known—or known was possible.

A random sampling of planets sounds like imaginative science fiction. One third of the surface of 55 cancri e is likely to be covered in diamonds, created by intense pressures and heat acting on its carbon-rich surface.

Then there’s TrES-2b, known as “the Dark Planet” because its surface reflects only 1% of the light that falls on it, making it darker than coal or black acrylic paint. Scientists don’t know why the planet is so dark, and the explanation may very well be something as yet unknown to science.

We’ve even found a whole group of Tatooine-like planets, including one called Kepler 16-b. This planet, like Luke Skywalker’s home, orbits two different stars. That means it would have two sunrises and two sunsets every day. Sadly, you wouldn’t be able to see those sunrises because, well there’s nowhere to stand. Kepler 16-b is about the size and density of Saturn—making it a gas giant, rather than the rocky planet young Skywalker inhabited.

Ok, we can’t actually travel to exoplanets yet, but NASA has created these fantastic travel posters so we’ll be ready when the time comes. Available to download here. ⓒ NASA/JPL-Caltech.

DOES ANYONE HAVE E.T.’s NUMBER?

The universe now appears to be crawling with all sorts of, well, other-worldly worlds. Could that mean it’s crawling with life as well? Are we alone? The honest and dizzying answer is that life is almost certainly out there somewhere, we just haven’t found it yet.

And finding it will be no small task. Exoplanets—small bodies who by definition are outside our solar system—are much too far away for us or our space probes to visit. Nor do we have the technology just yet to measure or image these worlds clearly enough to examine their surfaces for life.

Despite these current limitations, NASA Chief Scientist Ellen Stofan predicted in April of 2015 that microbial life on another planet would be found sometime in the next 10-20 years. (With the recent discovery of flowing water on Mars, that prediction seems closer than ever to being proved correct. And don’t forget about the promising situation on Jupiter’s moon, Europa.)

life is almost certainly out there somewhere, we just haven’t found it yet.

But what about intelligent life? With hundreds of billions of planets lurking in our galaxy alone, the odds are on that there is intelligent life out there somewhere. So far, most of the planets we’ve discovered are entirely unsuited for the development of even the most basic life, let alone intelligent life. However, there do appear to be some promising planets, and we’ve only just started looking.

BET ON LIFE

There is a famous equation first proposed by radio astronomer Francis Drake in 1961 that calculates out the probability that intelligent life exists in our universe. The equation multiplies out several semi-concrete variables such as the number of planets and the number stars, along with more speculative variables like the fraction of planets that have suitable environments in which life could develop. Even if you put in the most conservative estimates, the numbers still point to life.

Exoplanets have turned up bigger, smaller, hotter, and older than we predicted. Some of them are truly odd balls (of planetary rocks and gasses, that is). If they’re already surprising us, imagine what they can do once we can see and measure them better. Each new exoplanet discovery, each new technological breakthrough allows us to comb the cosmos ever more finely. That brings us one step closer to actually uncovering extraterrestrial life. It’s out there somewhere and we’re slowly training our eyes to see it.