Weighing Planets

At this month’s meeting of the American Astronomical Society, the team developing the NEID spectrograph at Kitt Peak National Observatory announced the instrument’s first light and released a great, processed image of the first spectrum that illustrates much of what I’ve talked about when giving behind-scenes-glimpses of the work. This is a spectrum of 51 Pegasi, which happens to have been the first star discovered to have an exoplanet back in 1995.

Credit: Guðmundur Kári Stefánsson/Princeton University/Penn State/NSF’s National Optical-Infrared Astronomy Research Laboratory/KPNO/AURA

The rainbow in the image above shows light from the star 51 Pegasi spread out by the spectrograph. To the left, you see the spectrum magnified so you can see more details. In particular, you see dark lines bisecting the rainbow in different places. These lines are caused when elements in the star’s atmosphere absorbs a little bit of the light. The dots above the lines come from a “calibration” image. They serve as a road map to tell you where you are in the spectrum. When a planet pulls the star toward us, those dark lines move a little bit toward the blue end of the spectrum (to the left in the image above). When the planet goes behind the star, those lines move a little toward the red end of the spectrum (to the right, in the image above).

What’s cool about this kind of measurement is that how far the planet moves those lines is directly related to how massive the planet is. If you measure the line movement precisely, you can measure how much the planet weighs. If you then use another telescope and take images of the star and watch for the planet to cross in front of the star, you can measure how much the planet makes the star’s light decrease. That tells you the diameter of the planet. With the diameter and the mass, you can calculate the density, which tells you whether you’re looking at a gas giant, a rocky world, a water world, or an ice giant world.

What’s more, I was on-hand when that first image was taken. We celebrated by pulling out a bottle of sparkling cider and toasting the instrument’s success. Afterwards, we got back to work characterizing and testing the instrument’s behavior. As you can tell from the image below, we have lots of people in the control room on these commissioning nights!

This past week has been especially fun as a science fiction writer and long time fan. We’ve been starting our nights by observing the star Tau Ceti, which appears in many science fiction novels, movies, and TV series. Among the notable novels where Tau Ceti appears are such classics as Isaac Asimov’s The Caves of Steel, Robert A. Heinlein’s Time for the Stars, and Samuel R. Delaney’s Empire Star. In Star Trek, Tau Ceti is known as the home of the doomed cargo ship, Kobayashi Maru. The system is the home of the planet Sea of the Morningstar in Bodacious Space Pirates, a wonderful anime series.

In fact, the star itself is very similar to the sun. It has a similar spectral type and a mass about 0.78 times the mass of the sun. It has four candidate planets in orbit and it’s a little less than 12 light years away, so it seems conceivable these are planet humans could eventually visit. I even gave it a cameo in the new, upcoming edition of my novel, The Pirates of Sufiro.