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TRAPPIST-1 Press Release

Frequently Asked Questions for Informal Learning Environments

For additional information, be sure to check out the NASA Exoplanet Exploration FAQ and Glossary.

• NASA Exoplanet Exploration FAQ - https://exoplanets.nasa.gov/faq/ • NASA Exoplanet Exploration Glossary - https://exoplanets.nasa.gov/glossary/

TRAPPIST-1 FAQ

1. What makes this discovery special?

There are many reasons why this discovery is truly special. The discovery of seven -sized in the same system has profound implications for our search for habitable worlds outside of our . On top of that, this is the first time that people have discovered a with more than one in the habitable zone that we can examine with . Transit spectroscopy is the measurement of a planet’s atmosphere while it transits its host star via studying how the light from the host star behaves as it passes through the planet’s atmosphere. By breaking up this light into its constituent colors via a spectrograph, we can learn about the chemical composition of the planet’s atmosphere.

Additionally, the reasonably low activity level of the central star means they are more likely to be able to support . Finally, the system is really close, only 39 light-years away. That makes it easier for us to learn more with our current observatories – Spitzer, Hubble, Kepler, and ground-based telescopes – and possibly learn the answers to more burning questions with the soon-to-come James Webb Space Telescope.

2. Why is this called the TRAPPIST system?

The TRAPPIST system is named after the TRAnsiting Planets and Small Telescope (TRAPPIST) robotic telescopes. The robotic telescopes are located in Chile and Morocco, but they are led by astrophysicists in Belgium. Because these telescopes discovered the first exoplanets in the system in 2016, the system was named TRAPPIST-1. The number 1 refers to the fact that this is the first system named after these telescopes. The program name (TRAPPIST) is a nod to the monastic order in the Belgium region, known by many for brewing Trappist beers.

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TRAPPIST-1 Press Release

3. How did we figure out there were more planets in the TRAPPIST-1 system?

We originally found planets in the TRAPPIST-1 system by looking at variations in light coming from the star. The light gets dimmer and forms a wiggle pattern when planets transit, or pass, between the star and the Earth. The pattern changes depending on how many planets there are, how fast they orbit, and other factors. NASA’s was able to observe the system for multiple transits at a higher precision than the TRAPPIST telescope could. So when the wiggle pattern — which was actually at least seven planets wiggling on top of each other — is observed long enough at high-enough precision, we can disentangle the signatures of the individual planets. Of course there could be even MORE planets hidden beyond our detection limit: their orbits may be too long, they may be too small to be detected, or they may be exactly in resonance with one of the existing planets we found.

4. Why was Spitzer looking at a system we already knew about?

The Spitzer Space Telescope was looking at the TRAPPIST-1 system because it could get *very* accurate masses, densities, and other properties that are impossible from even the stable ground-based observatory. The Earth's atmosphere limits our ultimate ability to detect these properties. It also limits our ability to detect the gases that might be present around an exoplanet (notably, .) Furthermore, Spitzer is an infrared telescope, so there are specific properties it can determine that cannot currently be observed elsewhere, even by NASA’s .

Once we knew there were planets, there was a lot to be gained by doing a deeper study with Spitzer to learn more about them. This discovery started with a ground-based facility - TRAPPIST – that looked at a large number of , searching for exoplanets. Then we pointed space-based facilities like Spitzer and Hubble on the TRAPPIST system because the TRAPPIST telescope found something interesting. Lots of astronomers compete for a limited amount of time to study objects with space telescopes. So space telescopes generally do not stare at a star for long periods of time looking for exoplanets unless another telescope has produced data indicating they might be there. We can’t look at everything with Spitzer/Hubble, but once we have a promising target, the case for using Spitzer or Hubble to observe a system becomes stronger. Spitzer just happened to find additional planets in the TRAPPIST system while it was observing the known exoplanets originally spotted by the TRAPPIST telescope.

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TRAPPIST-1 Press Release

General Exoplanet FAQ

1. How are exoplanets named?

Exoplanets are named after their parent star, and given lowercase letters based on when they are discovered and on their distance from their host star. For example, TRAPPIST-1A is the TRAPPIST-1 star. TRAPPIST-1b and TRAPPIST-1c were the first planets discovered around this star, with 1b being the closest to the star and 1c being the next closest. The third planet originally thought to be part of the system turned out to actually be three different planets. Since this was caught early, the new planets were named 1d, 1e, 1f, 1g, and 1h, going from closest to farthest. If we discovered an eighth planet orbiting TRAPPIST-1A five years from now, it would be called TRAPPIST-1i, regardless of its location.

2. What is an exoplanet?

The term exoplanet originally comes from extra-solar planet, which just means a planet orbiting a star other than our . Other stars are really far away. A star’s brightness makes it incredibly difficult to detect a nearby orbiting body. Given these observational difficulties, exoplanets were not detected until quite recently (the early 1990’s). Astronomers needed to create several ingenious techniques to find and study them. Now there are thousands of confirmed exoplanets, thousands more that are candidate exoplanets awaiting confirmation with more data, and future missions devoted to finding and characterizing them.

3. How many Earth-size planets have we found so far?

At the time of this discovery, there were approximately 340 confirmed terrestrial planets known. These are planets that are around the same size as the Earth (source: NASA’s Exoplanet Archive). This number comprises about 10 percent of all confirmed exoplanets. The other types of exoplanets are ice giants (like and Uranus), gas giants (like and ), and super- (planets that fall between Neptune/Uranus and Earth in size, of which there are no counterparts in our solar system).

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TRAPPIST-1 Press Release

4. What is the habitable zone?

Habitable means suitable for life. The one constant for life on Earth is the existence of liquid water. So, the simplest answer to this question is the habitable zone is the region around a star where liquid water can exist. If an exoplanet is too close, it will be too hot and water will evaporate. If an exoplanet is too far, it will be too cold and water will freeze. Given that stars come in a range of sizes and temperatures, the habitable zone will change depending on the type of star around which an exoplanet orbits.

Another common definition for habitable zone is where the average temperature falls in between the average on the surface of Venus and Mars, NOT accounting for atmospheric effects like Venus's runaway greenhouse. We do not yet know much about the atmospheres of most of the discovered exoplanets, and habitability is a complicated concept because an atmosphere greatly impacts whether or not the exoplanet is too hot, too cold, or just right for liquid water to exist. Including atmospheric effects are truly critical for actually understanding whether or not an exoplanet is habitable and will be critical in future studies of habitability.

Of course, being in a habitable zone does not guarantee habitability (e.g., the ). Being out of the habitable zone does not necessarily mean the planet or moon is not habitable (e.g., Europa, a moon of Jupiter that could have liquid water). But the habitable zone is a powerful indicator of where liquid water might most easily be found. Hence, the idea of a habitable zone is a guide for astronomers searching for places in our where life might exist.

5. How many Earth-size planets have we found in a habitable zone?

At the time of this discovery, there were six exoplanets labeled as “potential Earths,” based on their size and location within their systems’ habitable zone. The new discoveries bring the total up to nine.

6. Why do we look for exoplanets?

One of NASA’s big goals in the study of the universe is to address the question, “Are we Alone?,” by searching for life on planets around other stars. This question, pondered by humans for centuries, is just now beginning to be directly addressed, thanks to new technologies in telescopes and new strategies in searching for exoplanets. Before we can directly answer the question, we first have to find the exoplanets and begin to characterize their properties. For example, are they Earth-sized, in the habitable zone, and habitable as defined by the material in their atmospheres? These are the questions we are just now being able to answer.

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TRAPPIST-1 Press Release

7. What do we mean by Earth-like exoplanets?

“Earth-like” is commonly used to refer to exoplanets that are about the same size as Earth AND reside in the habitable zone.

As we learn more about exoplanets, we are able to use new data to reduce the number of exoplanets that may truly be Earth-like. In the TRAPPIST system, we have good information on the masses and sizes of the exoplanets. This information can provide a measurement of the bulk densities of the exoplanets. The bulk density (mass/volume) can tell us if the exoplanet is composed of rock, metal, water, or gas.

Ultimately, to truly tell if an exoplanet is Earth-like, we need to know if it can support life as we know it. To do that, we need to take detailed spectra of their atmospheres to discover what the atmospheres are made of. If we find exoplanet atmospheres with amounts of dioxide, , nitrogen, and water similar to the Earth’s atmosphere, we have a very good candidate for another habitable world similar to Earth.

It will most likely take larger telescopes to definitively find life. If astronomers can determine a molecule or a combination of molecules that are only known to come from biological sources, and we observe those molecules via transit spectroscopy, we may be able to find concrete evidence of life on exoplanets.

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