By Jonny Lupsha, Wondrium Staff Writer
Exoplanet research may be new, but its tools date back 400 years. Johannes Kepler, who died in 1630, found laws of motion that still matter to modern astronomy. NASA said 5,000 exoplanets have been discovered.
Emerging technologies and burgeoning research in the field of exoplanets is expediting scientists’ discovery of the exciting celestial bodies. NASA recently confirmed that through attentive stargazing, humans have found more than 5,000 exoplanets—that is, planets orbiting stars other than our own. But since many are too dim in the night sky to see, how are they found?
The stars around which these planets orbit move slightly differently based on how many planets are orbiting them. In his video series The Search for Exoplanets: What Astronomers Know, Dr. Joshua N. Winn, Professor of Astrophysical Sciences at Princeton University, explains how this practice has its roots in 17th-century astronomy.
Going around the World to Make a Point
Roughly 400 years ago, German astronomer Johannes Kepler developed three laws of physics, which are now known as Kepler’s laws of planetary motion. The first law is that a planet’s orbit is in the shape of an ellipse, not a perfect circle.
“The orbits of solar system planets are nearly circles, but they’re not exactly circles; they’re slightly squashed circles—ellipses,” Dr. Winn said. “Interestingly, the ellipse is not centered on the Sun, or even on the center of the mass: Rather, the center of mass is set off from the center of the ellipse. It’s a mathematical point known as the focus of the ellipse.”
He added that we quantify the angle of orbital ellipses on a range of zero to one. On this scale, zero would be a perfect circle, while the increase toward one would indicate more of a flattened ellipse. Earth’s orbit is rated 0.017. Since the Sun is a little off-center, and since Earth’s orbit of it is slightly elliptical, Earth technically gets closer to the Sun in January than in June.
“That brings us to Kepler’s second law, which says that whenever a planet gets closer to the Sun, it speeds up and when it gets further away, it slows down,” Dr. Winn said. “For planets like the Earth, on nearly circular orbits, this is only a slight effect that only astronomers ever notice. But as we’ll see, for some of the exoplanets, this can be a big deal.”
Kepler’s third law, Dr. Winn said, establishes a connection between the size of a planet’s orbit and the time it takes for the planet to complete one orbit. Obviously, the larger the orbit, the longer the journey. Also, the further from the star a planet is, the less it’s influenced by the star’s gravity, slowing the orbit further.
The Point Being…
All three of Kepler’s laws may not seem like they say much about finding exoplanets, since they detail planetary orbits of stars—and many, if not most, exoplanets are too dark in the night sky to see. However, they can be applied to the complementary physics of the stars themselves, which are quite visible.
According to Dr. Winn, we can do three important things with telescopes when we look at stars. First, we can measure which direction their light comes from, leading us to measure a star’s exact position in the sky. Second, we can measure the star’s color. Third, we can measure the star’s brightness. These three observations are called astrometry, spectroscopy, and photometry, respectively.
Since physicists have established that planets and stars orbit around one another, astronomers can use the “wobble” of stars to detect the number, size, and distance of exoplanets orbiting them. How? Although the process is a “daunting challenge,” in Dr. Winn’s words, we can observe how long it takes a star to complete one wobble. A European Space Agency telescope called Gaia is doing this.
“While Gaia is tracking all those stars to measure their parallaxes, it will sometimes see the additional wobbling motion due to the planets around those stars,” Dr. Winn said. “According to our best forecasts, Gaia will find thousands of exoplanets around nearby stars. So, we’ll get to learn their masses, periods, and the sizes and shapes of their orbits.”
Whew.