Introduction to exoplanets and how astronomers study them:
Where are we in space?
We live on Earth. Earth is a planet that orbits the Sun. The Sun is the only star in our solar system. When we look up at the night sky, we can see countless stars. There are billions of stars in our Milky Way Galaxy, but we can only see a few of them. There are billions of galaxies in the universe.
What are exoplanets?
We used to wonder whether the planets in our solar system were the only ones in the universe. We have learned so much since then! In the past few decades, astronomers learned that many of the stars we see at night are the centers of their own planetary systems, with one or more planets orbiting around them. We call the planets outside of our solar system extrasolar planets, or exoplanets.
In the mid-1990’s, scientists started finding ways to detect exoplanets orbiting distant stars. Since then, over 5,000 exoplanets have been discovered, and the list of exoplanet discoveries grows longer all the time.
What is the transit method of detecting exoplanets?
One of the methods astronomers use to study exoplanets is called the transit method, because as seen from our vantage point, the exoplanet passes in front of, or transits, the star. When this happens, the star’s light is dimmed by a tiny amount, and when the exoplanet finishes passing in front of the star it orbits, the star returns to its full brightness. The shorter the planet’s year is, the closer the planet orbits around its star, and the more often the star’s light is dimmed. Although exoplanets are too dim to be seen directly with our eyes, they cause a very small change in brightness to the stars they orbit as they pass in front of their star.
Some exoplanets are what scientists classify as “Hot Jupiters” because they’re large like the planet Jupiter, and they orbit very close to their star. Since the star is hot, and the exoplanet is close to its star, the planet is hot. Hot Jupiters have very short years, sometimes orbiting their star in just a few Earth days. Hot Jupiters are the easiest exoplanets to study, because they block more of their star’s light than smaller planets do, and you can see the change in brightness of their star very frequently.
When you participate in Exoplanet Watch, you can learn how to observe transiting exoplanets. Using small ground-based telescopes with tracking mounts and cameras, you can take a series of 50-100 pictures of a star as an exoplanet transits or crosses in front of it. With free software called EXOTIC, available on our website, we will show you how to turn this series of pictures into a light curve. Professional astronomers studying exoplanets use light curves to understand more about these distant worlds.
What are light curves?
A light curve shows the change in brightness of a star when an exoplanet passes in front of the star. We can't see the exoplanet directly using the transit method, but we can see the effect the exoplanet has on its star's brightness as it transits.
The horizontal part of the light curve is the baseline brightness of the star when there are no exoplanets passing in front of it. The dip in the light curve shows the star’s light blocked by the exoplanet during the transit. The deeper the dip, the more light is blocked, the bigger the planet. If you observe the same star over many nights, you can see how often the star’s light is blocked by a transiting exoplanet. The more frequently these transits occur, the shorter the year is for the exoplanet, and the hotter the planet is. By looking at light curves, you can even tell whether the planet has a thick atmosphere.
When we create light curves, we compare the brightness of the target star with the brightness of a few nearby comparison stars, or comp stars. It's important to choose comp stars that are not variable stars (stars whose brightness changes over time), so we use star charts from the American Association of Variable Star Observers (AAVSO) to help us identify stars with stable brighness to compare with our target star.
Are we alone?
One of the goals of studying exoplanets is to find another Earth-like planet. For all of human history, Earth has been the only planet known to have life. We want to know whether other planets could have life, too. By studying exoplanets, we can gain a better understanding of whether other places might have the conditions necessary for life to exist. Life on Earth requires water, energy, and the right mix of chemical elements. Finding other planets in our galaxy is the first step in answering questions about what’s out there to be discovered. Studying these planets further to see if they have the right conditions for life is the next step, and your involvement in Exoplanet Watch will help with both of these steps.
Now you have the basic background information you need to get started observing exoplanets yourself! Visit our Overview page next. If you have a telescope and you want to dive right in, you can go straight to our What to Observe page to learn which exoplanets will be transiting soon, or our EXOTIC page to process your observations with our free light curve generating software. If you don't have a telescope, you can request existing observations of exoplanet transits to use when creating your own light curves with EXOTIC using our data checkout system. If you have questions at any point in the process of making observations or creating light curves, reach out to us on Slack.
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