Upping the Ante on Earth-type Exoplanets

The age of the exoplanets, worlds around other stars, just gets more exciting every day. Today, astronomers announced that they’ve found seven terrestrial planets around a small, very cool star called TRAPPIST-1. Three of those worlds are int he so-called “habitable zone” where conditions can allow liquid water to exist. Rumors of this news have been kicking around for several weeks, but today was the confirming announcement.

The kicker? Temperatures on the surfaces of the three in the habitable zone are just low enough that they could contain liquid water on their surfaces. That covers two of the main requirements for the habitability for life: exist in the so-called “Goldilocks Zone” and have the right temperatures to allow liquid water. Mind you, this doesn’t mean they HAVE life, but even finding such worlds is a major accomplishment.

The discovery definitely is record-setting — it’s the most habitable-zone worlds found around a single stars beyond the Sun. The arrangement of the TRAPPIST-1 planets is also quite fortuitous. It’s possible that the upcoming James Webb Space Telescope could take a look at them and study their properties more closely. That data could reveal the signature of ozone if this molecule is present in the atmosphere of one of these planets. If it’s there, it’s theoretically possible it would be a tracer for biological activity on the planet.

How Were They Found?

The astronomy team announcing the work used different telescopes in Chile, Morocco, Hawaii, La Palma and South Africa to do the ground-based observations. NASA’s infrared-sensitive Spitzer Space Telescope joined the hunt last September. Using infrared detection is important because exoplanets give off infrared signatures that can’t always be detected from Earth.

Three of the planets were already known because they could be detected as they orbited their star. These are called “transit” discoveries, and kept astronomers busy doing more observations to determine their orbital periods. All the observations revealed that TRAPPIST-1 is a very interesting analog to our own solar system, but with seven terrestrial-type planets orbiting in what we would call the inner solar system. Three appear to be in that fortuitous habitable zone.

Modeling Planets

At least a few of those newly discovered worlds have densities very similar to Earth’s (that is, they’re more rocky and not like the gas giants). That’s why they’re called terrestrials. Computer modeling helped the astronomers understand the specifics of the system and predict what would be found.

In a press release today about this largest ever batch of such exoplanets yet discovered, the University of Bern (one of the partners in the work) explained more about these worlds and how they can be observed: Earth-like exoplanets orbiting dwarf stars are easier to observe than real Earth twins around solar-type stars. Since these dwarfs are also much cooler, the temperature zone that allows water to be liquid on the surface of the planet is much closer to the star. And, exoplanets that are close to their host star revolve more rapidly and produce more transits in a given timeframe. NASA held a press conference today to announce this momentous discovery.

Why We Hunt for Planets

The search for worlds similar to our own can help scientists understand how planets like ours form, and what conditions are possible for life. Although it may seem like a bit of a biased view, since we know that life arose on our world and we know what conditions under which it formed, we can look to other worlds to see if they have the same conditions. That’s not to say life can’t arise under other conditions. But, science is all about understanding and extrapolating conditions to figure out how things happen. The next steps (whether they are now or in the very near future) are to see if the assumptions we’ve made about our planet play out across other star systems and their worlds. News like today’s are part of an exciting tale that is still being told!