Protoplanetary Disk: Study Looks At Light For Clues To Exoplanet Mysteries

Study of a protoplanetary disk about 400 light-years from earth yields clues to the development of exoplanets. For the first time, scientists were able to use a technique called "light echo" to analyze the distance between the disk and the young star at its center.

A NASA illustration depicts a star that is surrounded by a protoplanetary disk composed of gas and dust. The disk is made up of material that is caught up in the gravitational force of a young star. Over millions of years, the protoplanetary disk will eventually evolve into clusters of material which will form planets.

Huan Meng is a post-doctoral research associates at the University of Arizona, and is the first author of the study that was recently published in the Astrophysical Journal. He is quoted in a NASA-JPL release.

"Understanding protoplanetary disks can help us understand some of the mysteries about exoplanets, the planets in solar systems outside our own."
The data on the study of a protoplanetary disk comes from the Spitzer Space Telescope, a NASA mission managed by the Jet Propulsion Laboratory at Cal Tech in Pasadena, as well as four earth-based telescopes observing the young star known as YLW 16B.
There is a gap between the protoplanetary disk and the star inside it and scientists in the study used the technique called photo-reverberation or light echo to measure that distance. The light energy emitted by a young star can fluctuate, flaring up as material from the disk occasionally finds its way to the surface. As the brightness flares up, some of the light energy is released and travels directly through space. Some of it hits the protoplanetary disk before bouncing out from the star, causing a slight delay.

By comparing the time it took for the light from the young star to reach earth and then the subsequent "echo" from the light that ricocheted off the edge of the protoplanetary disk, and using the speed of light as a constant, scientists were able to estimate the distance of the gap between the star and the surrounding disk. Astronomers have used the same technique to measure the accretion disk of supermassive black holes. An accretion disk contains the material that is being drawn into the gravitational pull of a black hole. This is the first time photo-reverberation has been used in the study of a protoplanetary disk.

As well as searching for earth-like planets, researchers want to find out more about so-called "hot Jupiters." In our solar system, Jupiter is a large planet composed mostly of hydrogen and helium gases – hence the label "gas giant" – but it is also very cold as compared to earth since it orbits the sun in the outer solar system. In other solar systems, astronomers have observed gas giants that orbit much closer to their own stars, making them much hotter. As reported in the Washington Post, one such hot Jupiter, dubbed KELT-fAb, was recently discovered to orbit a three star system.

As reported in Scientific American just a month ago, images from the Atacama Large Millimeter/submillimeter Array (ALMA), a Giant Radio Telescope in Chile, represent the best picture taken so far of a protoplanetary disk. The disk appears irregular in thickness, with gaps where scientists believe exoplanets are in the process of forming.

The ALMA image shows a gap in the disk at about the same distance from the star known as TW Hydrae as the Earth is from our own sun. Will the exoplanet that is evolving become a rocky planet like earth or a mysterious hot Jupiter? Scientists hope that further study will yield more clues on the forces at play in the evolution of a protoplanetary disk.

[Image via NASA/JPL-Caltech]