Is there a size limit on planets?

Jean-Baptiste Ruffio, research scientist in astronomy at the University of California, San Diego, checks in to find out.

Faculty Bio:

Jean-Baptiste Ruffio is a research scientist studying extra-solar planets in the Astronomy & Astrophysics department at the University of California, San Diego (UCSD). He has been developing statistical and instrumentation techniques to push the frontiers of planet detection and characterization with the largest telescopes in the world, both on the ground and in space.

Transcript:

How big can a planet be before it’s no longer called a planet? We used the James Webb Space Telescope to answer this question. In our own solar system, Jupiter is the largest planet. But astronomers have found much larger planets around other stars. In 2008, astronomers took the first picture of a surprising system called HR 8799. They saw four massive “maybe” planets orbiting the same star, each being 5-9 times the mass of Jupiter. We have tried to understand if they are planets or brown dwarfs. Brown dwarfs form like stars but they don’t get big enough to trigger the fusion of hydrogen. On the other hand, we believe planets generally from in a disk of dust and gas around young stars. Grains of dust collide and get bigger to form planetary cores. When a core gets big enough, it also pulls in large amount of gas to form gas giants like Jupiter. However, classical theories of planet formation struggle to explain the formation of the largest systems like HR 8799. In order to determine if they are truly planets or brown dwarfs, we can measure the composition of their atmospheres. Indeed, if we see a lot of icy and rocky material, that is the telltale sign of a planet. Brown dwarf would instead have a composition similar to the star. We therefore used James Webb to image the four planets around HR 8799. We then spent two years developing new analysis techniques to be able to measure the composition of these planets in unprecedented detail. We found that the planets had much more carbon, oxygen, and sulfur than their star. These observations finally provided the most direct evidence that HR 8799 is truly a planetary system and that planets can be much bigger than we thought.

Read More:

[UC San Diego Today] - How Big Can a Planet Be? With Very Large Gas Giants, It Can Be Hard to Tell

[Nature Astronomy] - Jupiter-like uniform metal enrichment in a system of multiple giant exoplanets

[ARXIV] - The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas

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Jean-Baptiste Ruffio, Unviersity of California, San Diego - How Big Can a Planet Be?

The Academic Minute

Jean-Baptiste Ruffio, Unviersity of California, San Diego - How Big Can a Planet Be?