Gaze up at the night sky on a clear evening. What can you see? The human eye can detect tiny points of light that we know to be stars, much like our own sun.
But with modern astronomy, we open the depth of our sight to tens of thousands of light years away. We can measure the temperatures of stars that would take eons to reach. We can determine the age of the universe simply by peering into its depths. And, we can even discover other planets.
Incredibly, out in our galaxy there are worlds that rotate and experience their own alien day and night. Their clouds meander over rocky terrain. They have atmospheres. They have surfaces. These planets are wild and strange; they may orbit around two or more stars or have lakes made of methane. Yet they retain eerily similar properties to our own Earth.
These are exoplanets. Due to their distance from Earth, we cannot directly see exoplanets. Yet, we are still certain of their existence. If we can learn more about these planets, we can learn more about our own origins, and could even discover extraterrestrial life.
Incredibly, we have already found hundreds of these distant worlds. And, we have methods of learning more about their makeup, such as mass, radius and even temperature. We can use these measurements to pinpoint earth-like planets that could potentially hold liquid water: a key ingredient for life on our own planet. What is most incredible is that we learn all this without ever actually seeing the exoplanet itself.
It all begins with a concept known as flux. Formally, flux is the rate of change per unit area of any moving substance. Light, for example, is a quantifiable and flowing substance made up of many photons zipping by at the mind-bending speed of 186,000 miles per second.
Imagine that you have two flashlights: a big flashlight and a small pocket flashlight. When turned on, the bigger flashlight has a larger flux of light than the smaller flashlight because it puts out more light. Something that is brighter has a higher flux of light than something that is dimmer. If you were to wave your hand in front of one of these flashlights, the flux of the light would decrease every time your hand passed by. A shadow is simply an area with a lower flux of light than a place with unimpeded light streaming through.
In order to apply this flashlight analogy to astronomy, imagine that stars are flashlights and an orbiting planet is like your hand waving in front of the light. When the planet blocks our view of the starlight, the disturbance is commonly referred to as a transit. Astronomers can observe the stars and detect the change in the flux of starlight and infer that an exoplanet is there. They don’t actually “see” the exoplanet during this event. They only detect its passing shadow via the decrease in the flux of light, like a hand blocking a flashlight. Scanning the stars and waiting for transits to occur is how astronomers find exoplanets.
After astronomers observe a transit, they can continue watching the planet to learn more. They can measure the orbital period, or how long the exoplanet’s year is, by timing how long it takes for the exoplanet to cross in front of the star again. They can find the radius of the exoplanet by detecting the size of its shadow. The radius can help determine if the exoplanet is large and gaseous like Jupiter or small and rocky like Earth. To see if these exoplanets bear more resemblances to Earth, scientists need to know the temperature.
The temperatures of exoplanets can be found using an astronomical event very similar to transits known as a secondary eclipse. A secondary eclipse is the opposite of a transit. Instead of crossing in front of the star, a secondary eclipse occurs when a planet goes behind the star.
Astronomers can detect when planets enter a secondary eclipse. This is because just like stars, planets emit light. So, when a star blocks the light coming from a planet, we can detect the change in flux. The difference is that planets shine in a different wavelength of light than stars do. Due to their lower temperature, planets emit light in the infrared wavelength. We can measure a secondary eclipse by using a telescope that detects only infrared light and pointing it at a star with a planet around it.
Exoplanets that are hotter will emit more light in the infrared than cooler exoplanets do. When there is a greater loss of infrared flux during a secondary eclipse, astronomers infer that the planet is hotter. This gives a fairly accurate reading of how hot the exoplanet is.
Secondary eclipses occur once for every orbit that an exoplanet makes. For some exoplanets, this can happen twice in a single earth day. For others, it can take much longer, sometimes multiple earth years.
Astronomers usually focus on the exoplanets with orbital periods that are ten days or less when searching for secondary eclipses. This way, telescopes won’t have to scan the sky too long waiting for secondary eclipses to repeat themselves. Because planetary orbits are essentially universal clocks, their timing can be found with extreme precision after only two secondary eclipses have been recorded.
Armed with knowledge of the size and temperature of exoplanets, astronomers can learn astounding facts. Scientists have discovered that exoplanets come in many varieties.
There are exoplanets that are so close to their home star that it only takes them less than a day to completely orbit the star. Gaseous exoplanets this close are often being sucked dry by their stars due to the intense forces. Rocky planets this close have entire seas of molten lava because it is too hot for any rock to remain solid. There has even been an exoplanet discovered that is made entirely of diamond. No two exoplanets are alike.
Along with strange exoplanets, there are many that are similar to the planets in our solar system. Just recently, NASA’s Kepler team, which has discovered hundreds of exoplanets, confirmed the existence of the most earthlike planet to date. Its radius is similar to Earth’s and it is the perfect distance away from its star to have the potential to hold liquid water. If we could get a secondary eclipse measurement of the planet, we would even be able to see if it is a similar temperature and has an atmosphere like Earth’s.
Unfortunately, it’s difficult to gather secondary eclipse data for small rocky planets, because they are too small to give off enough light. Big planets are more luminous because they are bigger, therefore their secondary eclipses can be easily distinguished from telescope error. Most secondary eclipse measurements have been taken on large gaseous planets like Jupiter. The technology to detect smaller bodies’ secondary eclipses is still advancing.
The future holds exciting possibilities. With this knowledge we grow closer and closer to finding Earth’s twin. Finding this identical planet could give us an understanding of how rare an earthlike planet is, tell us about how planets like ours evolve, and could even be the next step to finding extraterrestrial life.
New technology is making this even easier. The European Extremely Large Telescope is currently under construction, and once it is completed, this amazing telescope will have the capability to take direct images of exoplanets. For the first time in human history we could have a picture of a small earthlike world hundreds of light years away. Atmospheric composition and even traces of alien life could soon be standard data observed by telescopes.
What is truly amazing about all the information we can gather from exoplanets is that we’ve reached far beyond the conceivable scope of our tiny biosphere. Our small and insignificant species is expanding far beyond our limits. Like the ancient Greeks searching for answers, we look towards the sky; but, now, we can finally see.