Artwork of a star and exoplanet. Credit: ESA/ATG medialab
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Artwork of a star and exoplanet. Credit: ESA/ATG medialab

Two planets discovered: One by gravity, one by accident [Part 1]

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May 29, 2018

Astronomers recently found two new exoplanets — worlds orbiting other stars — and to be honest, while the planets themselves are interesting, what's also interesting is the way they were found. One was found using a weird property of gravity, and the other was found by accident!

It turns out that describing how this all works would make a marathon-length article, so I've broken it up into two parts. Today is Part 1: a planet found through the dust due to its gravity.

The exoplanet is called UKIRT-2017-BLG-001 — breaking that name down, UKIRT is the United Kingdom Infrared Telescope, 2017 is the year it was found, and BLG-001 means it was first planet found by the telescope near the galaxy's central bulge using what's called gravitational microlensing.

I've described this technique before, but in a nutshell, the path light from a distant object takes to get to Earth can be diverted if there's a third, massive object in between us. The gravity of this object bends space, which can have all sorts of weird effects on the light coming from the more distant object. You can get multiple images of it, or the image can be smeared out, or the brightness can be magnified by quite a bit.

It's that last one that counts here. Suppose you're looking at a very distant star. If another star passes between us, the light from the more distant star can get brighter over time, then fade as the interloper passes. If that in-between star also has a planet orbiting it the planet's gravity can have the same effect, though on a smaller scale. This overall effect is called gravitational lensing, and it happens a lot when you look out into the Universe at distant galaxies. When something smaller like a star or planet does it, it's called microlensing.

This is a rare event, so the best way to see it is to look toward a crowded part of the sky. The center of the galaxy is great for that! There are lots of stars in that direction (it's like looking downtown toward bright city lights), so the chances of seeing a lensing event are higher. So astronomers using UKIRT looked toward the center of the Milky Way, taking lots of observations of as many stars as possible to see if any changed brightness in the right way. Doing this in the infrared is a big advantage too, since that kind of light can pierce through the thick dust in that direction, allowing even more stars to be seen.

The field of stars around the microlensed star that led to the discovery of UKIRT-2017-BLG-001. The lensed star is in the center of the green crosshairs, and the yellow box contains stars used to determine the extinction toward the star.

The field of stars around the microlensed star that led to the discovery of UKIRT-2017-BLG-001. The lensed star is in the center of the green crosshairs, and the yellow box contains stars used to determine the extinction toward the star. Credit: Shvartzvald et al. 2018

In 2017 they got their first hit! The star that was lensed is about 36,000 light-years away. That puts it on the other side of the galactic center, though the uncertainty in that distance measurement is large; it could still be on this side of the center.

The star and planet doing the lensing are much closer, about 20,000 light-years away. From the shape of the graph showing the brightening over the event (what astronomers call a light curve), the best fits indicate the star has about 0.8 times the Sun's mass (and is therefore an orange dwarf, cooler than the Sun) and the exoplanet about 1.3 times the mass of Jupiter. The planet orbits the star about 600 million kilometers out, so a bit close than Jupiter orbits the Sun.

There are plenty of assumptions that go into these numbers, including the planetary system's age, the amount of dust between us and it (though IR light can get through dust, it's not 100%, and that can affect the results), and the distance to the lensed background star. So the uncertainties in these numbers allow for some wiggle room (for example, the star could be between 0.6 and 1 times the Sun's mass), but most likely the star and planet are pretty close to being as described.

The “light curve” of the background star, the brightness versus time. The big peak is due to the gravitational lensing of a star passing between us and the background star, and the smaller peak just after due to a planet orbiting the interloping star.

The “light curve” of the background star, the brightness versus time. The big peak is due to the gravitational lensing of a star passing between us and the background star, and the smaller peak just after due to a planet orbiting the interloping star. Credit: Shvartzvald et al. 2018

Making this even more difficult is that the amount of dust they're seeing through changes a lot, and changes rapidly depending on where they are looking. There are lots of small clumpy dust clouds in that direction, as well as long filamentary ones, so the extinction — the amount of light absorbed by dust — changes a lot from star to star. On top of that, it changes depending on the distance to the star! A star farther away will tend to have more dust blocking it.

So to find more examples of this, the astronomers have their work cut out for them. This planet is just the first they've found in their survey, and they'll find more. Not only that, but a proposed space telescope, WFIRST, will dedicate a big chunk of its time to looking for exactly these kinds of microlensing events toward the galactic center, and may find thousands of planets.

assuming it gets built. It's a high priority for NASA, but Trump zeroed out its funding in his 2019 NASA budget. Congress, for their part, has supported WFIRST, so there's hope. I'd love to see it built and operating; a wide field infrared survey observatory would be a great complement to the James Webb Space Telescope, which has a much narrower field of view.

This new exoplanet is telling us that there are hidden treasures buried deep in the dust of our galaxy, and we already have a map with a big X on it (hmmm, literally, too). All we need are the tools to dig them up.