By Steven Gimbel, Ph.D., Gettysburg College
There are possibly many life-bearing planets outside our solar system. The question is how to find them without having to travel through space. Planets are small in astronomical terms and give off no light. Optically, we can’t see exoplanets, the planets around other stars. Could there be other ways of detecting them?
Gravitational Wobbles of Faraway Planets
One approach to finding planets orbiting other stars is by trying to observe gravitational effects. Strictly speaking, the Earth does not revolve around the Sun. This is not to refute what Copernicus declared, but to emphasize what Newton’s theories really showed us: that the Sun and the Earth both revolve around a common point—the center of mass of the two-bodied system. If the Sun and the Earth had the same mass, that point would be mid-way between them, but since the Sun is heavier, the center of mass moves along the line connecting them towards the Sun.
Indeed, the Sun is so much heavier that the center of mass of the Earth and Sun system is actually located within the Sun itself. As a result, when the Earth moves around this point, it moves around the Sun. But when the Sun moves around it, it’s revolving around a point inside itself, but not at its center.
The result is a wobble, much like someone keeping a hula hoop going around his or her waist. The heavier the planet, the bigger the wobble in the Sun. We can look for these solar wobbles optically, that is, if a planet is big enough and the star is close enough, we might be able to see the movement over time.
Learn more about Big Bang cosmology.
Measuring Doppler Shifts
A better way is to spot that movement is as the result of Doppler shifts. We can check for changes in the light the star sends out. That’s how the first exoplanet was discovered. Pulsars are stars that emit a strong beam of light. As the star rotates, this light flashes on and off like the light on top of a police car. As a result, we can get a good idea of the star’s rotation.
This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, Wondrium.
The Polish astronomer Aleksander Wolszczan and the Canadian astronomer Dale Frail noticed that the behavior of a pulsar star they were observing was a bit askew. Their calculations showed that this had to be the result of two planets orbiting the pulsar, and so the age of exoplanets was launched. The exoplanet they discovered would not be hospitable to life, so the search for Earth-like exoplanets began in earnest.
Larger planets like Jupiter are called gas giants, and they were the first kind of planets found. These Jupiter-type exoplanets are more easily seen because of their large size, but again these are not the sort of places in which life as we know it could be found. So, other, subtler physical effects of smaller planets were sought.
Another way to see planets is in transit. Transit is the name astronomers give the phase of a planet’s motion in front of the star it orbits. For example, there are times when we can observe the transit of Venus across the perceived disk of our Sun. If viewed at the right time in the right way, a black spot seems to move across the surface of the Sun. That spot is not on the Sun, but rather is Venus moving between us and the Sun, thereby blocking the light.
With an exoplanet, the amount of light blocked would be small, and the dark spot too small to see. But if we had a sensitive enough instrument that could detect a slight decrease in the brightness of the star, we could infer the transit of the planet.
Between the wobble and the transit methods, astronomers have now identified about 1800 exoplanets. We had feared at one time that planets were rare, but the most recent estimate is that our own Milky Way contains about 400 billion planets, and that’s just our galaxy!
Learn more about the origins of life.
Planets in the Goldilocks Zone
Just finding planets isn’t enough though. We need planets that are Earth-like, and that orbit their stars at just the right distance, in what scientists call the habitable zone. Too far out and there wouldn’t be enough heat to sustain life. Too close and the heat would be too intense. This zone is also called the Goldilocks zone because it needs to be neither too hot nor too cold, but just right.
Right now, we’ve found 21 confirmed exoplanets that are of the right sort and in the right range of orbit, and another 100 or so that are candidates, that is, whose existence, in particular are not yet nailed down.
There’s Tau Ceti e, a mere 12 light-years from Earth, all the way out to Kepler 283 c which is 1740 light-years away. The name Kepler refers to the Kepler space observatory, a telescope in orbit used to get images that would be difficult with a land-based instrument.
Perhaps the most tantalizing result we have gotten from Kepler is the existence of the planet Kepler 186 f, which orbits the red dwarf star Kepler 186. It’s virtually the same size as the Earth—only about 1.4 times larger—and has an orbit about the same distance out as Mercury. Since its sun is smaller and dimmer than our sun, there is a chance that the amount of heat it receives could be just right.
Is there life out there? With so many planets and so much space, the chances are that the right sort of planet, the right distance away from the right sort of star would have to be found out there somewhere, or perhaps lots of somewheres. But where is that somewhere? We’ll keep looking in hopes that when we find it, we’ll have to completely redefine reality once more.
Common Questions about the Search for Exoplanets
If an exoplanet is big enough and the star is close enough, we might be able to see the movement over time. A better way is to spot that movement is as the result of Doppler shifts. We can check for changes in the light the star sends out.
The astronomers Aleksander Wolszczan and Dale Frail noticed that the behavior of a pulsar star they were observing was a bit askew. Their calculations showed that this had to be the result of two planets orbiting the pulsar, and thus the first exoplanets were discovered.
The most common type of exoplanets located are large Jupiter-size exoplanets which would not support life as we know it.
The Goldilocks zone is another name for the habitable zone around a star, where it is neither too hot nor too cold, but just right to be able to sustain life as we know it on exoplanets.