Astronomers Use "Light Echo" To Measure Size Of Star's Disk

An artist's impression of star YLW 16B. NASA/JPL-Caltech

Scientists have used a rather novel technique to measure the size of the planet-forming disk of dust and gas surrounding a star near Earth. Using two telescopes, astronomers measured the time it took for light to travel from the star YLW 16B to Earth, and the time taken for the “echo” of this light to bounce off the disk.

This technique has allowed astronomers to calculate the distance of the star to its disk, which they estimate at 0.08 astronomical units (AU, 1 AU is the Earth-Sun distance), or 8 percent of Earth’s distance to the Sun.

This finding, published in the Astrophysical Journal, will allow us to get a better understanding of how planets form in these protoplanetary disks. Such disks are known to produce planets around stars, as objects scoop up material and sweep out gaps. But exactly how some planets form, such as hot Jupiters – gas giants that form extremely close to their stars – remains a mystery.

"Understanding protoplanetary disks can help us understand some of the mysteries about exoplanets, the planets in solar systems outside our own," said lead author Huan Meng from the University of Arizona in a statement. "We want to know how planets form and why we find large planets called 'hot Jupiters' close to their stars."

Above, a graphic showing how the measurement was made. NASA/JPL-Caltech

To make the measurement of this gap required a bit of trickery from the astronomers, though. At 400 billion light-years away, it’s difficult to make a direct visible observation because of the dust and gas surrounding the star.

However, YLW 16B is known to be a variable star, which means that it is often and irregularly changing in brightness. This is because it is still near the start of its life, having formed just 1 million years ago (compared to 4.6 billion years for our Sun). When the star has one of its outbursts, we can detect this change in brightness.

But the change also means that the “echo,” produced by light bouncing off the protoplanetary disk, will be noticeably different. Using ground-based observatories, the team were able to measure the light from the star, and 74 seconds later, they used NASA’s Spitzer space telescope to measure infrared light from the disk's echo. This gave them their measurement of 0.08 AU for the gap.

The astronomers now think this method could be used to measure disks around other stars, giving us a better grasp on how weird and wonderful planets form.

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