Xinyu Dai: We found a population of planets in the lens galaxy

Xinyu Dai
Xinyu Dai

A University of Oklahoma astrophysics team has discovered for the first time a population of planets beyond the Milky Way galaxy.

Xinyu Dai, professor in the Homer L. Dodge Department of Physics and Astronomy, OU College of Arts and Sciences, with OU postdoctoral researcher Eduardo Guerras, made the discovery with data from the National Aeronautics and Space Administration’s Chandra X-ray Observatory, a telescope in space that is controlled by the Smithsonian Astrophysical Observatory. Until this study, there has been no evidence of planets in other galaxies.

A paper, “Probing Planets in Extragalactic Galaxies Using Quasar Microlensing,” by Dai and Guerras on this study has been published in the Astrophysical Journal Letters, a leading journal in astrophysics.

Professor Dai gave an exclusive interview to Mediamax about his study.

Professor Dai, why did this direction attract you so?

Gravitational microlensing is  such a powerful tool to study the background objects (supermassive black holes in our case) in detail and foreground object as well (lens galaxy and its content). We are able to study properties of objects that cannot be directly measured in the near future. As we build better and better telescopes, we can always use microlensing on top of them to probe even further objects.

No evidence of planets in other galaxies has been found until this study. What exactly did you intend to find out when you started?


Our original goals are to constraining the environment around supermassive black holes that are actively accreting and how fast supermassive black holes are spinning. However, during the research, we found that a persistent microlensing signal we observed can only be interpreted with the presence of a population of planet mass objects in the lens galaxy.

What scientific method did you apply to conduct your research?

We used gravitational microlensing in our research. Gravitational lensing is a phenomena predicted by Einstein based on the general theory of relativity. An object with a gravitational field can focus light just like an optical lens.

Could you describe the object that you have found?

We found a population of free floating planets in the lens galaxy of RXJ1131-1231 system. These planets are not the traditional bound planets that orbit stars, but they move between stars. This population of free floating planets has been detected in the Milky Way a few years ago. Now, we have detected them in a galaxy 3.8 billion light years away. We constrain the planet-to-star number ratio to be 2000 objects in the Moon to Jupiter mass range per star, or 200 in Mars to Jupiter mass range per star, or 1 Jupiter per 10 stars. We can estimate the number of stars in that galaxy, then multiply these factors, and we estimate the total number of planets in the galaxy is more than a trillion.

How will you develop this discovery in your further research?

We are working on a few other galaxies and try to detect the planet population. We are also developing other methods on better constrain the mass spectrum of the planet population.


What was scientific community’s feedback to your discovery?

In general, the feedback is quite positive. I have received many positive responses from the scientific community either discussing the details of the model or implications of the findings, and congratulations as well. Some experts expressed skepticism concerning if a different population of objects can reproduce the observed data. I think if they run the calculations through a supercomputer, the skepticism will go away. That is what we have done!

How important is this discovery for you? What role will it play in your further research?

I think it will be one of highlights of my career when we look back 30 years in the future. My team will spend more efforts on this new, uncharted area in the next few years.

Marie Taryan talked to Xinyu Dai


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