OU researchers locate first known planets in another galaxy

Image of the gravitational lens RX J1131-1231 galaxy with the lens galaxy at the center and four lensed background quasars. It is estimated that there are trillions of planets in the central elliptical galaxy in this image.

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NORMAN, Okla. — There was a time when the nine planets in our solar system (counting Pluto) were the only ones we knew existed.

That changed a little more than two decades ago, and since the turn of the second decade in the 21st Century, we now know there are thousands of planets in our galaxy alone. And now, thanks to two University of Oklahoma astrophysicists, the map is getting bigger.

Using a technique called microlensing, Professor Xinyu Dai and Eduardo Guerras, a postdoctoral researcher, have discovered the first known planets outside of the Milky Way galaxy. They were found in RX J1131-1231, a galaxy 3.8 billion light years away from Earth.

“More than two decades ago was the time when the first planet outside of the solar system was discovered,” Dai said. “That’s a significant discovery back then. But the field has grown so much. Here, we just expand the boundaries, the frontiers.”

Dai said he and Guerras had the idea to start looking at the objects later to be known as planets about six months ago. They began analyzing data gathered by the Chandra X-ray Observatory, a NASA space telescope controlled by the Smithsonian Astrophysical Observatory.

While examining what the telescope had found, Dai said the data was turning up some abnormal readings. So they decided to look at it more closely with the microlensing technique.

The method is based on Albert Einstein’s work. When light hits objects in space, it is bent around those objects, similarly that light is bent as is passes through a lens, Dai said.

“Any mass with a gravitational field can bend light,” he said. “So a massive object can act like a big lens.”

So microlensing looks at this on a much smaller scale, since the objects were so far away. Using the OU Supercomputer Center for Education and Research, Dai and Guerras ran through equations to compare the light bending around specific objects in the distant galaxy to the light that bent around known stars.

“We think of a magnifying glass that magnifies light in the background and allows us to see clear,” Dai said. “The background is so clear and so magnified, then we inferred there must be something magnifying.”

The process took several months of checking and re-checking their work, but it boiled down to one possibility: the objects had to be planets. Dai and Guerras had a paper on the discovery published in “Astrophysical Journal Letters.”

“It took quite a long time, several months, to finish these calculations,” Dai said. “And we also need to check these calculations. At the beginning, there were some mistakes. We checked them and indeed, these small mass objects explained the puzzling data we had.”

Dai said the planets are unbound, moving between the stars in the galaxy and not part of a star system similar to our own. This lines up with what astronomers have found in our own Milky Way.

Based on the ratios of stars to planets Dai and Guerras calculated, there could be trillions of these planets in this galaxy alone. The planets they located range in size from as small as the Moon to as large as Jupiter, and there are 2,000 of them for every one star.

If the lower end of the scale is set to Mars-sized planets, the ratio is 200-to-1, Dai said. Future work will include applying this technique to other galaxies.

“There are other galaxies that have this kind of signature,” Dai said. “We suspect we’ll be able to detect objects in those galaxies, as well. In addition, we want to use other types of signatures. Magnification and changing of the background color of an object are just some signatures of microlensing.”


Troxtell writes for The Norman Transcript, a CNHI News Service publication.

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