Planet orbiting a distant star, Credit: NASA

Worlds Around Other Stars

From the moment I picked up my first astronomy book and discovered that the stars were like our sun, only very far away, it seemed obvious to me that planets around other stars must be commonplace, many of them surely harboring life. There were so many stars in the sky, it was hardly likely that our situation was unique. Yet in those days, not a single example of an extrasolar planet had been found, so it was still a matter of speculation.

In an episode of the Cosmos TV series, Carl Sagan informs a group of school children that within their lifetimes we should be able to figure out whether planets exist around nearby stars.

“That will happen in your lifetime,” he predicted. “And it will be the first time in the history of the world that anybody found out, really, if there are planets around the other stars.”

Over six thousand such exoplanets have since been discovered, of the several tens of billions that are believed to exist in the Milky Way galaxy. There are numerous techniques for detecting exoplanets, but the one illustrated above involves measuring how the brightness of a star drops as the planet moves in front of it – an event known as a “transit.” If an exoplanet’s orbital plane is somewhat aligned with our line of sight to the star, then we should see the star’s light grow slightly dimmer at the beginning of the transit, brightening up again when the transit is complete.  Exoplanet Watch is a citizen science project open to anyone who would like to participate. By taking part in the program, you can help fine tune the available data on exoplanet transit times allowing scientists to make more efficient use of space-based telescopes such as Kepler and TESS.

For us Earth-bound astronomers, the process is called differential photometry. That’s because we’re measuring the light from the target star and comparing this with data from other reference stars in the same part of the sky. Since an exoplanet transit affects only the light from the target star, we can discard any data in which we see also see a dimming of the comparison stars, as that was likely due to atmospheric noise.



  • Here's an example of a transit light-curve taken from our observatory. Up to this point, I’ve used a color camera with nothing but a IR/UV filter in front of it. My process is basically as follows:

    ★ Capture a series of 30-second exposures through the night. Make sure the target star is visible from your location for the entire duration of the transit. Use the NASA transit-finder to get an estimate of the start and end times, and then add a buffer of couple of hours on each side of that time window just to be safe.

    ★ I then calibrate my images, and convert them from color to grayscale.

    ★ Solving your images is important – meaning all the data files must contain exact coordinates, so that all your exposures can be properly aligned.

  • ★ I then take note of the coordinates of my target and comparison stars, and enter this information into NASA’s photometry software, EXOTIC.
    ★ Finally, I run the EXOTIC process, which can take several hours before it produces a light curve and associated data file. Hopefully, I end up with a nice light curve indicating the presence of a transiting exoplanet.

  • This is an image of K2-18, a red dwarf star that was discovered by the Kepler spacecraft to have at least two planets going around it.

     


  • One of those planets, K2-18b, is worthy of special note, as it was the first exoplanet found in the habitable zone of its star - an area in which conditions are favorable for life. More specifically, the habitable zone refers to an area around a star in which a planetary surface might be expected to have liquid water, which is deemed essential for life forms to evolve.

    You cannot see the planet itself, only its home star which is visible just below the big star in the center of the image. I've labeled it for you, but you'll need to zoom in to see it. The planet is larger than Earth and is 124 light years distant from us. In fact, the photons of light that entered my telescope and impacted my sensor to create this image - they set out on their journey just as the 1901 World's Fair was opening in New York.

What's Next?

For future exoplanet observations, I’ll be switching to a mono camera with filters appropriate to the target. With any luck, that should help to reduce the noise and get a clearer signal from some of the fainter targets out there. Just as Sagan predicted the discovery of exoplanets, I would say that within the lifetime of anyone born after 2010, there is good chance that they will see the first direct imaging of an Earth-type planet around another star, along with a clear indication of extraterrestrial life. As a planetary inhabitant myself, I find that prospect very exciting!