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Worlds Without End

Think we might eventually travel to other planets? In a way, we already are.

It might sound strange to you, but a telescope is no less a vessel of exploration than the ones used by ancient explorers to discover…By Lisa Kaltenegger

It might sound strange to you, but a telescope is no less a vessel of exploration than the ones used by ancient explorers to discover new continents. Telescopes give us glimpses of places that are unlike anything we have seen before, and take us to places that seem impossibly bizarre. But these distant worlds are as real as anything here on Earth. The light a telescope collects also transports us through time, millions and even billions of years into the past. Out of all the fascinating places in the universe I want a telescope to take me to, the newly discovered worlds among the stars are at the top of my list. Traveling to another world—not just another country or another continent, but to a whole new world—is an amazing adventure. And if it so happens that we humans are not alone in the universe, then these worlds—those planets and moons—are probably where that life will be found, whether it is similar, or very different, from anything we know of here on Earth.

The simplest astronomical instrument is the human eye, which is sufficient to see our neighboring planets. When you look up at the night sky in a dark area, you’ll see at least 5,000 bright points—and on some nights many more. These are stars, or other suns, which are really too far away from us to see them as a disk on the sky, like we see our own sun. But among these thousands of bright lights are also a few planets, bright points that move rapidly during the night. Brightest in the night sky, and closest to the Earth most of the time, is Venus. Venus is very similar to the Earth, with about 80 percent of Earth’s mass and 95 percent of its diameter. Venus might also have had an ocean like Earth’s at one time. But if it did, this ocean was lost into space. Its water was evaporated by increasing surface heat, transported all the way to the top of its atmosphere, and broken by radiation into hydrogen, which escaped into space, and oxygen. Today Venus’ surface temperature is a balmy 863 degrees Fahrenheit, its atmosphere is 96 percent carbon dioxide, and it is shrouded in caustic sulfuric acid clouds that prevent us from seeing its surface in visible light. But these reflective clouds also make Venus very bright. It is so bright that ancient cultures called it the Morning Star or Evening Star—though it is by no means a star.

To see the dimmest planet in our Solar System, Neptune, you’ll need to catch more light than you can with your naked eye. We’ll need “bigger eyes”: binoculars, or even better, a telescope. When we look through binoculars, we can see the eighth and farthest planet from the sun. With a 4 to 6-inch telescope you can even see its colors on a good night, a blue hue around its disk. A storm on its surface the size of the Earth forms and reforms every few Earth years, and appears as a great dark spot. Though it is stormy, Neptune is also one of the coldest places in the Solar System, with temperatures down to about 55 degrees Kelvin (−360 degrees Fahrenheit). Its 14th moon was just discovered, on July 15 2013, using the Hubble Space telescope.

A telescope is no less a vessel of exploration than the ones used by ancient explorers to discover new continents.

Before we move out of our Solar System, lets adjust our perspective. If we shrink the Sun to the size of a small sugar grain, then all the planets out to Neptune fit into the size of an Oreo cookie. Now take that cookie in your hand and look for the next star around you. It’s about two football fields away! Light needs about four years to travel that distance, while it needs only eight minutes to get from our Sun to us.1

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How do we see such distant planets? The humble telescope can take us there, if we know what to look for. In 1995, we found our first gas giant exoplanet around a sun-like star. The observation was based on detective work that looked for the tiny wobble of stars caused by unseen planets orbiting around them. That first planet, which we called 51 Pegasi b (see exoplanet map), is around 50 light years away. You can see its host star  with your naked eye. It orbits its sun in only four Earth days. This means it is close to its Sun, and hot—believed to be 1300 degrees Kelvin at its surface. Because of its proximity to the sun, its size, and its composition—it is mostly made out of helium and hydrogen-—we call this type of planet a “hot Jupiter.” Swiss astronomers found it using a 2-meter telescope at the Haute-Provence Observatory in France, along with an instrument that allowed very precise spectral fingerprints of the star to be measured (the ELODIE spectrograph). The star’s wobble translated into a spectral wobble, which was picked up by the instrument.


Let’s go to another gas planet, now, called HD 209458b. This one is not too different from 51 Pegasi b, and is only a bit farther away from us (about 150 light years). But it’s important because of the way we found it. Rather than looking for the wobble of its star, we looked for its shadow. If we stare at the brightest of the billions of points of light in the sky using telescopes as small as 18 to 40 centimeters (7 to 16 inches), we can occasionally detect a slight, periodic dimming. This results when, strictly by chance geometric alignment, a planet crosses our line of sight to the star it is orbiting. This temporarily and partially blocks our view of that star.

HD 209458b was found in 1999 using this so-called transit method. It’s a hot giant planet roughly the size of Jupiter, and orbits its star once every 3.5 days, reducing the light from this star by only about 2 percent. That fast orbit means it, like 51 Pegasi b, is very close to its star: about 4.5 percent of the distance between Earth and our sun. It also makes it easier to see, both because we don’t have to wait long to notice the periodic dimming, and because the bigger the exoplanet is, the more light it blocks (which is why we found big planets first). By comparison, the Earth dims our sun’s light by only 0.01 percent. An alien civilization would need a much bigger telescope to see us this way, or it would need to use a space telescope to escape atmospheric distortions.

Now we’ve visited two gas giants. But what we are really interested in is rocky planets, because they are more likely to host life. To visit the first detected rocky exoplanet, called CoRoT-7b, we need to travel 500 light years. It was found using a 27 centimeter diameter (10.5 inch) telescope, in orbit around the Earth. CoRoT-7b is also quite close to its star, making its transit easy to find. In fact it is so close and therefore so hot that it probably has rivers of lava flowing on its surface. It needs only a bit more than 20 hours to orbit its Sun, which appears a few hundred times bigger in its sky than our Sun does in ours.

Planets like CoRoT-7b (and others like it, including Kepler-10b and 55 Cancri e) are still too hot to support life as we know it, with a surface temperature that would melt rock. What we’re really interested in is cooler, more hospitable planets. To get to these, we have to observe for quite a bit longer. The Earth transits the sun only once a year, not every 20 hours. So an alien astronomer would need to observe us for several years to catch at least three transits—the first one to say there is something there, the second one to confirm that the object is orbiting a star, and the third one to confirm an orbit prediction and reveal whether other planets or moons are tugging on the planet. If the star is smaller, a planet needs to be closer to get the same radiation, and therefore orbits faster. This is why the first cool, potentially rocky planets we found orbit around cool red stars (like Gliese 581 d, Kepler-62e, and Kepler-62f).

To look for cooler planets (literally), let’s abandon our Earth-bound telescope for something a bit fancier. The Kepler telescope is a specially designed telescope with a very large field of view: 105 square degrees, comparable to the area of your hand held at arm’s length. Kepler has a kind of tunnel vision. It stares at the same star field for its entire mission, continuously and simultaneously monitoring the brightness of about 150,000 stars. It has a large diameter, too, of 0.95 meters. This reduces the noise inherent in photon counting, and allows it to measure the small change in brightness of a transiting planet about the size of Earth.

An alien astronomer would need to observe us for several years to catch at least three transits.

Kepler lets us move closer to smaller, cooler, and (as far as life goes) more interesting planets, rocky, and with radii smaller than twice the Earth’s. We are especially interested in planets whose distance from their star allows for liquid water on their surface. Such planets are in a region called the habitable zone. The extent of the habitable zone (and the temperature of the planets it contains) is controlled, not just by starlight (which must not be too bright or too dim), but also through a geochemical cycle that regulates the level of carbon dioxide in the planets’ atmospheres. Earth’s version is the carbonate silicate cycle. Such cycles keep the temperature throughout the habitable zone above freezing, adding and decreasing greenhouse gas concentrations as necessary to keep the temperature consistent, like donning or doffing a sweater next to a bonfire.

Kepler has recently found the first two rocky transiting planets—one 40 percent, the other 60 percent larger than Earth—within the habitable zone. Both planets are in the Kepler-62 system, which is about 1,200 light-years away, and contains a minimum of three other planets, all closer to the star. But we don’t necessarily need to travel this far to see an interesting planet. The huge distance is the result of a rather more prosaic reason, which is that Kepler is a statistics mission and needed to scrutinize a part of the sky with many stars in it, to gather sufficient data. This in turn required it to look at distant parts of the sky, which contain a greater number of stars for a given field of view. Even at light speed, 1,200 light years is quite far. If Charlemagne had taken off in a light-speed vessel, he would be just arriving there now!

Kepler was just a start for us. NASA just selected for launch in 2017 the Transiting Exoplanet Survey Satellite (TESS). TESS is in a way an “all-sky” Kepler. It looks for small transiting exoplanets around the closest and brightest stars over the whole sky, rather than singling out one narrow field. TESS will scan the entire sky in its first two years, one year for the Northern sky and another for the Southern sky. It will take us to planets that are in our solar neighborhood, rather than thousands of light years away. These planets will be bright, either because they are so close by, or because their star is bright. That means we will be able to examine their atmosphere, once we have built the next generation of telescopes.

Getting close enough to an exoplanet to discern its atmosphere is an exciting prospect. When a transiting planet blocks our view of its star, part of that star’s light gets filtered through the planet’s atmosphere. This produces a characteristic absorption spectrum. The light we measure shows missing chunks of energy, which are used to excite atoms and molecules in the planet’s atmosphere. The biggest telescopes we currently have, like the twin Keck telescopes in Hawaii with their two 10-meter mirrors, or Hubble with its 2.4-meter diameter, can already use this spectrum to detect some of the gases in the atmospheres of hot Jupiters. To do similar observations for small, rocky, Earth-like planets, we need bigger telescopes, like the 6.5-meter James Webb Space Telescope that will follow Hubble, or the new big ground telescopes with 20-meter to 40-meter mirrors that should be operational by 2025.

Hidden in these atmospheric signatures could be telltale signs of life breathing in and out. For an alien looking at Earth from far away, this telltale spectral fingerprint would be a combination of oxygen (or ozone), a gas that reacts with oxygen (like methane or any other reducing gas), and water. Therefore, we look for the same signatures on other planets. The presence of a reducing gas would convince us that the oxygen we observe is not just sticking around because it has nothing to react with. If oxygen is reacting with a reducing gas, then something (hopefully life!) has to produce oxygen continuously, and in big amounts, to replenish it. We have not found a way for geological processes to do so. So the combination of these three gases—oxygen, ozone, and a reducing gas—becomes the telltale sign of life.

Once we know what the atmosphere of a planet consists of, and how fast it spins, we can think about its weather patterns, and even what the environment might be on its surface. Huge, explosive volcanoes that spew sulfur dioxide high into the stratosphere of other worlds could be detectable, allowing us to study their geological history and compare it to Earth’s. In other words, we could start doing comparative planetology. The changing spectral fingerprint of Earth, based on our own geological records, could give us insights into what is happening on worlds light years away.

Hidden in these atmospheric signatures could be telltale signs of life breathing in and out.

This is what we might have in store for us. But as we look to the future, we are reminded of our most audacious planetary probe. Voyager 1 started its journey in September 1977, carrying ten instruments that, at the time, were breakthroughs. The two Voyager missions discovered 22 new satellites, the Uranian and Neptunian magnetospheres, active volcanism on Io, the rings of Jupiter, and large-scale storms on Neptune. Each Voyager mission carried a gold-plated audio-visual phonograph record, including photos of Earth, the sounds of waves breaking on a shore, and Blind Willie Johnson.

But, perhaps most remarkably, in 1990 after it had completed its mission to characterize the outer Solar System, Voyager turned around and took a breathtaking picture of the Earth. For the first time, we had a glimpse of how we—or another Earth—would look from billions of miles away: a pale blue dot, suspended in space and looking tiny and somehow lost in the vastness of its surroundings. Yet Earth is home to an incredible diversity of life forms, some of which look up at the stars wondering if there could be life on any of the other planets out there. To this day, the Voyager picture of Earth remains the most distant photograph ever taken of our planet.

We know now that Earth is just one among billions of planets in our Milky Way. As we discover more rocky planets in habitable zones, and build more powerful telescopes to read their spectral fingerprints, we find ourselves on the verge of something truly momentous: finding another pale blue dot. 


Lisa Kaltenegger is a Research Group Leader at the Max Planck Institute for Astronomy, and a researcher at the Harvard-Smithsonian Center for Astrophysics.

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