Astronomy Extra Credit


Introduction and background

The Kepler spacecraft and its successor TESS depends on discovering exoplanets by fortunate shot arrangement. On the off chance that the orbit of a remote planet just so happens to converge our perspective of its parent star. At that point, the earth will every so often cross our viewable pathway, causing a modest yet quantifiable overshadowing. This is an obvious dunk in the brilliance of the star that uncovers the nearness of the planet.

Most solar systems won’t have such fortunate arrangements, so these missions invest a great deal of energy gazing vainly at loads of stars. In addition, these traveling strategies uncover one-sided demography of the universe. To more readily build the odds of a successful arrangement, it’s best if the exoplanet is near its star. On and off chance that the planet is far away, it must be extremely fortunate for its orbit to fall along our viewable pathway.

Major Findings of the Talk

One Shift, Two Shift

Except in sporadic cases, we don’t ever actually get to see the stars wobble and wobble back and forth under the gravitational suggestions of their exoplanets. But we can see the light from those stars, and moving objects will shift their light. The same way a siren shifts in pitch up and then down as the ambulance races past you, light can turn redder or bluer depending on its motion: a light source moving towards you will appear ever-so-slightly bluer, and a receding light looks a tiny bit redder. So even though we can’t see the star in motion, we can detect the small change in its light pattern as the planet causes it to swing closer and farther from us. This method works best when the earth is directly along our line of sight (just like with the transit method), but it can also give a detectable signal when it’s not perfectly aligned. As long as the star has some decent amount of back-and-forth in our direction, the light will shift.

Of course, the stars themselves are in motion through space, causing a general light shift, and reliable measurements are difficult to come by since the spherical surfaces are roiling, boiling cauldrons – not exactly the best source to get precise measurements of motions. But the regular, rhythmic, repeated movements due to the influence of an orbiting planet stick out in a pronounced way, taking the form of a characteristic curve, even if we haven’t observed the system for an entire exoplanet orbit.

The impression of the Talk, Learnt Lessons and Critiques

Double-Check the Exoplanets

That’s not to say that this method (called by various fun technical names such as “radial velocity” and “Doppler spectroscopy”) is perfect and instantly unlocks all the scientific secrets of an alien world. Far from it. Like any other technique hanging from the science tool belt, there are shortcomings and limitations. For one, the shifting of light alone isn’t enough to fully reveal the details of the exoplanetary orbit. As we see a relatively small planet perfectly aligned with our line of sight. Or a much bigger planet with a tilted orbit? Both cases would lead to the same signal.

What next?

With the hundreds of candidate exoplanets in the bag using the radial velocity method, how many of them also transit in front of their star? More specifically, now that we’ve seen a planet once with one technique, can we catch it again in a follow-up with something like the TESS mission? Not only would a follow-up confirm details of the planet like density and radius, but it would also uncover new ones.  What’s more, these kinds of cross-checks are crucial to help reveal hidden biases and weakness in the respective methods. Do radial velocity and transit methods always agree on the properties of the exoplanets they find? If not, why not? To better use the ways independently, we have to examine the results when they’re used simultaneously carefully.


Unfortunately, we can’t expect too much planet-hunting crossover. A recent study ran the numbers: starting with hundreds of candidates tagged with the radial velocity method, only a couple dozen should also be lucky enough to be transiting. Of those, just about a dozen will be measured by TESS during its two-year observing run. And of those, only about three will be never-before-seen transits. While that’s not a lot of samples, what precious data we get will still be invaluable to future searches and future understanding of our exoplanetary neighbors.