This post is from Jeff’s Space News E-mail, 3/17/17
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Why Should You Care About TRAPPIST-1?
You probably saw news reports about a planetary system called TRAPPIST-1, which was recently discovered to have SEVEN Earth-size planets in orbit of a single star. Three of those planets orbit within the star’s habitable zone — which means they orbit at a distance where there is a good chance that these planets could have surface oceans of liquid water. The story got a lot of attention because it sounds so cool… but I think there are even more important reasons why this is a momentous discovery. I’ll put it the way I did in a recent visit to an astrobiology class at UCLA…
How many of you think that our entire world is a flat disk, covered by a dome-shaped sky?
[No hands go up.] Glad to see this response! But you should realize that this means you disagree with what most of our ancestors would have answered until just a few thousand years ago, since the flat Earth and dome sky is what we see if we never venture too far from home. This model of the universe only lost favor as travelers reported seeing different constellations when traveling north or south, proving that there is more to the sky than just what we see locally. This and other observations gradually led the ancient Greeks to develop their Earth-centered model of the universe.
How many of you think that Earth is the center of the universe, surrounded by a great celestial sphere?
[No hands go up.] Good, because this isn’t correct either. But historically, this means that you are answering differently than nearly anyone who ever lived up until about 400 years ago. Because it was only then, with what we call the Copernican revolution, that we finally learned that Earth is not the center of the universe.
A brief aside: I’d argue that despite the fact that we now know we are not the center of the universe, we can trace most of the world’s ongoing problems to people who still haven’t taken this fact to heart. Instead, they suffer from what I call “center of the universe syndrome” (COTUS for short), meaning that they behave as though they believe the universe revolves around them.
How many of you think that Earth is the only known world in the universe that has the right size and orbital distance to have surface oceans of liquid water?
[No hands go up.] Good, but that one was tougher, right? Because up until just a couple years ago, Earth was the only such planet known. There have been a handful of other discoveries, but you can now begin to see why TRAPPIST-1 is so important: While we don’t yet know for sure that any of these worlds actually have oceans, they seem to have the basic characteristics necessary for oceans to exist.
How many of you think that Earth is the only world in the universe with life?
[A few people tentatively hold up hands.] I see that most of you expect that life exists elsewhere, but some of you are not so sure. And now you can see the key point. Ever since the Copernican revolution, most scientists have assumed something called the “Copernican principle,” which in essence holds that we are not special in any particular way. That is, just as we learned 400 years ago that Earth is one of many planets orbiting the Sun, the Copernican principle suggests that there should also be similar planets around other stars, at least some of them with life and perhaps even civilizations.
We still don’t yet know for sure whether the Copernican principle is correct, but TRAPPIST-1 has brought us to the doorstep of the answer. Not only does it suggest that other Earth-like planets are likely, but the fact that there may be three of them in a single star system suggests that such planets will prove to be quite common. Indeed, it is not a stretch to imagine that our galaxy alone might contain 100 billion such planets – so many that it would take you more than 3000 years just to count them out loud! (It’s true: it would take at least 100 billion seconds to count 100 billion stars out loud, and 100 billion seconds is more than 3,000 years.)
Moreover, TRAPPIST-1 is relatively nearby, at only about 40 light-years away. Keep in mind that this is still really far away: On the scale of the Voyage model solar system, where the Moon is only about an inch and a half from Earth, the distance to TRAPPIST-1 would be about 10 times the distance across the United States. We won’t be going there any time soon. However, this distance puts it within reach of spectroscopic observation with future telescopes, including the James Webb Space Telescope slated for launch next year, which could in principle allow us to recognize spectroscopic signatures of life.
In other words, if any of the TRAPPIST-1 planets have life, there is a good chance that we’ll know about it within the next couple of decades. If that happens, we’ll have finally reached the culmination of the Copernican revolution, with the understanding that we are just one part of a vast cosmic web of life.
Another aside: Some people assume that this knowledge would diminish our own status in the universe, but I think it does the opposite: It would confirm that, using only the power of our bodies and brains, we are capable of figuring out our true place in the universe. This would be a remarkable achievement, regardless of whether beings on other worlds have done it before us. It would also confirm that we have created something important here on Earth, giving us all the more reason to make sure that we preserve it for future generations.
Why is it called TRAPPIST-1? It’s an acronym for the project that discovered it. See the TRAPPIST-1 web site, where you’ll also find much more detail about the system and its discovery.
I heard the planets might not be hospitable because of synchronous rotation; is that correct? Maybe… Because the star in this system is much smaller and dimmer than our Sun, the habitable zone is located close enough to the star that we expect the planets to be in synchronous rotation, meaning that just as our Moon always shows the same face to Earth, these planets would always show the same face to their star. This would seem to imply that one side of the planet would become quite hot and the other quite cold, leaving only a narrow zone in between where temperatures would be “just right.” However, this neglects the effects of atmospheric circulation, which some models suggest could even out the temperatures globally. The planet Venus offers evidence for this possibility: Venus rotates very slowly and therefore has very long days and nights, but its temperatures are about the same everywhere due to redistribution of heat by its atmosphere. So bottom line: This might or might not be an issue, and my own personal guess is that it’s not.
You called it the habitable zone, but new reports call it the “Goldilocks zone”; which is it? I’m glad you asked. Here’s what I wrote about this jargon issue in my book On Teaching Science: This term [Goldilocks zone] has recently become popular as a way to describe the region around a star in which an Earth-like planet (e.g., a planet with liquid-water oceans on its surface) could conceivably form. Both scientists and the media now routinely use this term over the formerly favored term of “habitable zone,” presumably because it’s kind of cute to think of the region in which a planet could be habitable as the “just right” region around a star in the same way that Goldilocks found the baby bear’s porridge, chair, and bed to be “just right” in the English fairy tale known as “Goldilocks and the Three Bears.” The problem, however, is that not everyone knows the story of “Goldilocks and Three Bears”; in fact, it’s rarely known to people whose native language is not English, and often unfamiliar to students with immigrant parents. Given that one of our goals is to increase the diversity of students entering science, it’s crazy to introduce a new term that will make no sense to them when we have a perfectly good term (habitable zone) already. (For more of my tirades on jargon, see the excerpt posted here.)