[Artwork depicting the TRAPPIST-1 planetary system by ESO/M. Kornmesser/spaceengine.org]

Big exoplanet news today: Astronomers have found a whopping seven planets orbiting a nearby star!

Not only that, but the planets are all around the same size as Earth, and some are orbiting at the right distance from their star to possibly have temperate surfaces. Holy wow. Everything about this story is cool.

Literally. The host star is called 2MASS J23062928-0502285 (from the 2MASS survey of cool stars, with the numbers being its coordinates on the sky), but everyone now calls it TRAPPIST-1; the planets were discovered using the TRAnsiting Planets and PlanetesImals Small Telescope, or TRAPPIST. It was the first star using that telescope discovered to have planets.

Three of these planets were announced in 2016. They were discovered using the transit method, the most successful way to find exoplanets we currently have. In principle, it’s simple: If a planet orbits a star, and we see that planet’s orbit edge-on, then every time the planet gets between the star and us it blocks a tiny bit of the star’s light. If we carefully measure the amount of starlight we see over time, that dip can be seen.

In practice, it’s not that easy! Stars are big and planets small, so the amount of starlight blocked is usually at most around 1%. That means very careful measurements must be taken. Also, there are lots of things that can masquerade as planets: star spots (sunspots on other stars), background stars that change brightness and mess up the observations, stellar activity, and more.

If you want background info, I explain all this in the exoplanets episode of Crash Course Astronomy:

Video of Crash Course Exoplanets

TRAPPIST-1 is a very small, cool star — it’s what we call an M8 red dwarf. It only has a mass 8% of the Sun’s and a radius 10% of the Sun’s (that’s only a wee bit bigger than Jupiter!), and shines at a feeble 0.05% of our own star. That’s a mixed bag of news for planet hunters. The star is faint, so it’s hard to detect small variations in its light. Happily, it’s close by — just under 40 light-years— so, even though it’s intrinsically weak, it’s close enough to see.

On the plus side, its small stature means a given-sized planet blocks a bigger percentage of its light. If an Earth-sized planet transits a Sun-sized star, about 0.1% of the light is blocked. But if that same planet transits a Jupiter-sized star like TRAPPIST-1, it blocks about 1% of the light. That makes the dip in light much easier to detect.

Zoom In This plot shows the light curves (the change in the amount of starlight detected) during the transits of each of the seven TRAPPIST-1 planets. The time is measured in minutes, with 0 being the center of the transit. Note that the bigger planets have deeper dips, and the ones farther out from the star take longer to transit. Credit: ESO/M. Gillon et al.

This is where things get very interesting indeed: If you know the size of the star, then you can calculate the size of the transiting planet by seeing how much light is blocked. It depends on the relative areas of the star and planet. A planet 1/10th the radius of the star will block 1/100th the light. A planet ½ the radius of the star will block ¼ of the light, and so on.

Judging from their transits, the three planets initially found in 2016 were around the same size as Earth. But the astronomers found that the timing of the transits kept changing a bit, and that was very exciting: That meant there may be more planets in the system, their gravity tugging on the three known planets, changing their orbital timing a little bit.

With that clue, they kept looking, and sure enough, they found four more planets for a total of seven! The planets are named TRAPPIST-1b, c, d, e, f, g, and h, in order of discovery (but, as it happens, also in order outward from the star). They range in size from just smaller than Earth to somewhat bigger.

Zoom In The relative sizes of the TRAPPIST-1 system. The Sun is on the lower right, and TRAPPIST-1 is depicted between it and Jupiter. The TRAPPIST-1 planet sizes are in the red box, compared to the four terrestrial planets in our solar system below. Jupiter's moons are included in the white box. Credit: ESO/O. Furtak

But we can find out even more. The length of time it takes a planet to transit is related to its orbital speed and, therefore, distance from the star (a planet farther out moves more slowly, and therefore has a longer transit). Now, get this: All seven planets orbit the star much, much closer than Mercury orbits our Sun. The inner planet, 1b, has an orbit just 1.5 Earth days long! It literally circles the star once ever 36 hours or so. Even the outermost planet, 1h, has a period of 14 – 25 days, so it takes less than a month to orbit the star once.

You might think that would scorch those worlds, but remember that TRAPPIST-1 is a dim bulb with a surface temperature of just 2300°C (4100°F). The planets are actually getting decent heat from the star despite its feeble output because they’re huddled so close to it. The actual temperatures of the planets are not possible to know at this time but, using computer models, we can estimate them given guesses at the planets’ compositions, sizes, atmospheres, and so on. Different models yield different results, but at least three of the planets appear to be the right distance from the star to have surface temperatures allowing liquid water to exist on the surface.

I want to be very careful here: We only have estimates on the planets’ sizes, not direct measurements. The masses were found by looking at how the transit times of each planet changed as the planets tugged on each other. So, we don’t know their actual masses very accurately yet, or whether they have atmospheres, or what they’re made of. All we know for sure are their sizes and orbital periods. So, everything else is extrapolated from that. Beware of articles in the media saying we’ve found Earth-like planets! We actually don’t know for sure that any of these planets has a temperature like ours, or even if they have air. We can say very little about how habitable they are.

Still, though, this is very exciting news! The fact that this planetary system is just 40 light-years away is great news, too. It means that follow-up work with bigger and more powerful telescopes could reveal much more information. For example, the James Webb Space Telescope, due for launch next year, could very well be able to look at the individual planets in this system to check for signatures of atmospheric composition like ozone. If that’s found, then that tells us a lot more —in that example, the existence of oxygen in the planet’s atmosphere.

But it’s too early to say anything about that, yet. But don’t be disappointed. Be the opposite: This is nothing short of amazing. When I was a kid, we only knew of nine planets (and now 8, or maybe soon over a hundred depending on how you’re counting) in our own solar system. Then, in the early 1990s, the first planets were found orbiting another star, and just a few years later the first ones found orbiting a Sun-like star. Now, due to advances in technology and the desire to find these planets, we know of thousands of exoplanets, alien worlds orbiting alien stars. We see them in all sizes, some big, some tiny, some with air, some likely without, some in multiple planetary systems, some near, some far.

We’ve found enough to come to one of the most startling conclusions in modern science: Planets may outnumber stars in the galaxy! Not every star has a planet, but many stars have multiple planets (like the Sun, or TRAPPIST-1), and so the galaxy itself may have hundreds of billions of planets.

Hundreds of billions.

How many of these are like our own blue-green world? We’ve seen a handful that could fit that bill, but we’ve only just started searching. Our tech is getting better, our methods more sophisticated, and we’re finding more planets almost literally every day.

TRAPPIST-1 is a wonderful milestone in this search. It’s telling us that all kinds of stars can have planetary systems, and may have multiple planets like ours. And mind you, this star is nearby and one of the most common kinds of stars in the galaxy. The more of them we search the more systems like this we’ll find.

The sky is buzzing with planets. If you needed a little bit of good news today, then carry that around with you. This is the sort of thing we discover when we look up, get curious, ask questions, and then try our hardest to answer them.

The payoff is understanding the Universe, and having our wonder amplified by it.