A Side of Science: Planet Hunting and Black Holes

In my latest video, I talked about how NASA’s TESS recently observed a star shredding black hole. But, TESS stands for “Transiting Exoplanet Survey Satellite” — meaning, it looks for exoplanets (planets outside our solar system). So, what was this planet hunter doing looking at black holes?

It turns out that TESS’s observing strategy is perfect for doing a lot of different types of astronomy, not just finding planets.

Illustration credit: NASA’s Goddard Space Flight Center

TESS searches for exoplanets by having its four cameras stare at large swaths of the sky for about a month at a time. These swaths are called sectors, and are shown in the animation above as long, skinny rectangles. There’s two special regions — one each in the Southern and Northern Hemisphere — where one camera essentially stares at the same patch of sky as the different cameras move each month. This is called the “continuous viewing zone,” and TESS stares at it for nearly a month at a time.

Illustration credit: NASA’s Goddard Space Flight Center

TESS is looking for planet transits — these are dips in the light we receive from a star due to a planet blocking part of its star’s light as the planet passes in front of it. Seeing one dip isn’t enough to say that it was caused by a planet or something orbiting the star, so by staring for a month at a time, TESS may catch additional dips in light, depending on the orbital period of the candidate planet. The more dips it sees, the more it can constrain the orbit and other planetary characteristics (and the more confident the researchers are that it is a planet they’re seeing), but that’s only possible for planets with short orbits.

Exoplanets with longer orbits will only be observed a couple of times during the month that TESS observes (or once, or not at all….but in those cases, TESS may be on to another observations before another orbit occurs). Unless, of course, that planet in the continuous viewing zone — then TESS has the opportunity to detect planets with much longer orbits.

Video credit: NASA Goddard Space Flight Center

This observing strategy, though, works for more than just exoplanets. There are a number of things that happen suddenly in the sky: a star blows up in a supernova or a star gets eaten by a black hole, for example. These are events that we can’t predict. Sure, there are some stars that are likely “close” to becoming supernovae (I’m looking at you Eta Carinae!), but “close” in astronomical terms can still be thousands to millions of years.

By keeping an eye on the sky for a long time, TESS has a high probability of catching these unpredictable events and getting data not only from the event, but before the event happens — something we can’t do with telescopes that turn to the event after it starts.

And, in case you missed my video on the star shredded by a black hole caught by TESS, ground-based telescopes and other NASA missions, be sure to check it out:

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