Astronomers measure the air of a super-Earth exoplanet

June 5, 2023   Issue #573

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Astronomy News

It’s a big Universe. Here’s a thing about it.

Our solar system is weird in some ways.

That’s hard to know without other solar systems to look at, but as it happens we know of lots. Exoplanets — alien worlds orbiting alien stars — abound, and well over 5,000 such planets are now confirmed.

The most common kind of planet seen around other stars, however, doesn’t exist in our own solar system. These are planets with a mass between Earth’s and Neptune’s (which has about 17 times Earth’s mass). As far as we know there’s no such world in our solar system (though the still-hypothetical Planet Nine might be one), but we see them around lots of other stars.

One such is the wee red dwarf GJ 1214, a dim, cool star just under 50 light-years from us. And I do mean dim; it shines with less than 1% of the Sun’s luminosity. Still, it’s close enough that we can get decent observations of it, and in late 2009 a planet was indeed found orbiting it. Called GJ 1214b, it’s just under 3 times the diameter of Earth, and is a mere 2.25 million miles from the star. That’s close enough to heat it substantially, as we’ll see in a sec.

We see the planet’s orbit edge-on from Earth, so once per orbit it passes directly in front of the star, creating a mini-eclipse we call a transit. Half an orbit later it passes behind the star, what’s called the secondary eclipse (the transit is the primary one). This is in fact how the planet was discovered, since during primary eclipse the amount of starlight we see drops by a fraction.

During the transit, some of the star’s light passes through the planet’s atmosphere, and that’s critical: Different molecules in the air absorb very specific colors of light, betraying not only their presence but their identity. That means we can actually determine what’s in the planet’s atmosphere even from hundreds of trillions of kilometers away!

But there’s more. As the planet circles the star, it goes through phases just like the Moon does. At transit, when it’s between the star and us, we see the unlit side, so it’s new. A quarter orbit later and we see it half lit. Just before and after secondary eclipse, when it’s behind the star, we see it fully lit. Then a quarter orbit later it’s half lit, and then back to new.

That means in principle we can see the planet’s brightness change as it orbits the star… and that’s precisely what new JWST observations of GJ 1214b have shown! [Link to paper]

The top plot shows the total brightness of the star + planet (called a light curve), and you can see the big drop when the planet is in front of the star (shown in the middle, what’s called Phase 0). The smaller dips at the beginning and end of the observations are when the planet goes behind the star, and its light is blocked.

The bottom plot zooms in on the brightness. The dots are individual observations, and the red line is made using a physical model of what’s going on, inputting things like the star brightness, the planet size, temperature, and so on. You can see the big dips again, but note the overall brightness isn’t constant! It goes up and down in a smooth curve, because the planet is undergoing phases. The brightest peak is just before secondary eclipse, when we see the planet as “full”. The dimmest is just before transit, when it’s new.

To be clear, we can’t physically see the planet separate from the star; it’s too close. All this is inferred from the light curve, where we see the combined light form the two.

A lot can be understood from this. For one thing, these observations were done using MIRI, a camera on JWST that sees in thermal infrared light. So these observations are actually measuring the planet’s temperature. Given the change in brightness over time, the astronomers found that the dayside of GJ 1214b is about 280°C (535°F) and the night side cooler at 165°C (325°F).

That’s cookin’! Here’s the fun bit: Hydrogen is really good at transferring heat from the day side to the night side, so if the planet had an atmosphere of all hydrogen the day/night temperature swing would be low. The fact that it’s well over 100° means there must be heavier stuff in the air there. The astronomers think there’s a lot of water and/or methane in the atmosphere there, judging from the planet’s spectrum, with a thick layer of aerosols (small particles suspended in the air) covering it. The overall reflectivity of the planet is about 50% which is pretty high. That means it reflects half the star’s light it receives, making it whitish or shiny. Earth reflects about 40%, for example, and Venus over 90%.

By the way, if you look at the phase diagram you can see the peak in the planet’s brightness is actually a few hours before secondary eclipse, and starts to drop before the actual eclipse. That’s because the hottest/brightest spot on the planet is not directly under the star, what we’d think of as where it’s noon and the star directly overhead. Winds probably blow the hottest air a bit to the east, so that shifts when we see the peak brightness.

I know this can all be a bit difficult to digest, so let me take a step back and summarize: JWST observed a planet 500 trillion kilometers from Earth and from those measurements astronomers can determine how hot the planet is, where it’s hotter and cooler, at least in part what the atmosphere is made of, and how reflective the planet is.

Holy wow. Mind you, again, we can’t see this planet directly. In images it would be overwhelmed by the star’s light. But tiny changes (less than 1% or so!) in brightness reveal nearly all these wonderful data about it.

So we’re learning about these planets in between Earth and Neptune, ones that are simultaneously weird (because we don’t have one) but also totally normal (because they’re the most common kind of planet in the Universe). And by doing this we’re putting our own solar system into cosmic contrast, learning about the Earth in the process in a way we never could if we only studied it by looking down.

I am constantly amazed by what we humans can do when we desire to understand things. We may be stuck on Earth for now, but our urge to explore takes us a very, very long way indeed.

Et alia

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