it is particularly challenging to make direct measurements of exoplanets—planets orbiting other stars—, but direct measurements tell us a lot about a planet’s orbit, composition, and evolutionary history. these measurements are crucial for placing our own solar system in a broader context and understanding our place in the universe.
most planets form in protoplanetary disks, pancakes of gas and dust that are left in orbit around young stars once the star itself has been formed. as small particles of dust stick together, they form pebbles, which collide and form boulders, and on and up to asteroids, “planetesimals,” and then rocky planets like our own. the most massive of these planets can start to pull large quantities of hydrogen and helium gas onto their surfaces, and balloon into ice giants (like Neptune or Uranus), and even further into gas giants like Jupiter and Saturn. These giant planets have cores of rock and ice, but large shells of pure gas. These two layers begin to mix and smash together, until the gas giant’s atmosphere is swirling with a cocktail of hydrogen, helium, and heavier elements and molecules, like water vapor, carbon monoxide, methane, and little particles of rock and dust, like quartz. As the planets are forming, the gas in the protoplaentary disk that surrounds them drags on them over the course of their orbit around the star; like walking against a headwind on a blustery day. This forces young, massive planets into relatively circular orbits; think of our solar system: for the most part, all the planet’s have circular orbits, because (we think) of this initial formation within a disk.
in my latest paper (published on the pre-print site arXiv today, and by the Astronomical Journal in a few weeks), i took a closer look at a nearby, young, gas giant exoplanet. before i had taken these observations (late last year) we didn’t know what the orbit of this planet looked like, or exactly what it was made out of. i wanted to understand a few key things about this planet: how circular is its orbit? how many heavy elements are in the planet’s atmosphere? what is the planet made out of? these things could tell me whether this planet formed similar to how we believe our own gas giants, Jupiter and Saturn, did. (see my previous blog about my last first author paper, for a related investigation).
i used the very large telescope interferometer in the atacama desert of chile (see pictures here) to resolve the light from the planet separately from its bright host star. our observations showed that the planet’s orbit is circular, maybe even more circular than Jupiter’s. this indicates to us that the planet probably formed in a protoplanetary disk that put gas drag on the planet’s orbit – it is otherwise unlikely that a randomly captured binary system of this separation would be perfectly circular.
the light was sent through a fiber optic cable and passed into a spectrograph that split the light up into its component colors. this is all infrared light, naked to the human eye. the light showed a pattern consistent with methane in the planet’s atmosphere. this, combined with other measurements, told us about the ratio of heavier elements (carbon, oxygen, iron, etc) to hydrogen and helium in the planet’s atmosphere.
then, we created models of the planet’s atmosphere and compared these with our data. we found that the current models have a few deficiencies (poor assumptions about the clouds in the atmosphere, for instance) that result in incorrect results, but we were still able to roughly measure the heavy element content of the planet’s atmosphere. the planet has about as much metal as you’d expect if it formed from the “bottom up” as pebbles, then asteroids, then planetessimals collided together, and then got mixed in with a bunch of hydrogen and helium.
so, this planet af lep b formed “like a planet should.” talk about a tautology! really, though, these measurements have been so historically difficult to make that even proving that certain exoplanets orbit in circles or are composed of a certain fraction of rock vs ice vs gas have been impossible. we’re talking about taking pictures of fireflies next to lighthouses, after all. so, being able to prove that this planet that is 27 parsecs, or about 90 light years away, from our own Earth formed like how we imagine the planets in our own solar system formed is a big deal to astronomers like myself.
the incredible precision of these measurements is really my secret to success, and i haven’t covered that in detail in this post. instead of using just one telescope to take an “image” we combined the light from four telescopes. the “GRAVITY” instrument takes the light from the four telescopes at the VLT and “interferes” the light together, similar to the double slit experiment but in 2 dimensions and with 4 apertures. this interference creates a wavey pattern centered at the location of the planet on the sky; that makes it much easier to distinguish the planet’s ligth from the bright host star.
once we’ve isolated the wave pattern of the planet’s light as seen through the four telescopes (the image below), we can model the wave and measure very precisely where the planet is located (and split its light up into different colors using the spectrograph, to measure the planet’s composition as described earlier).
in the future, we hope to use this instrument to observe a lot more planets and create a statistical sample. that will tell us how common circular or oval shaped orbits are, and therefore how normal or strange af lep b or our own solar system truly is.
p.s. astronomy wikipedia editors fast – someone has already updated the page for AF Leporis with the key results of my paper. this will probably be superseded by other papers soon. for now, i’ll enjoy my 15 minutes of citation fame
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