Episode for August 17th, 2023 Download

Exoplanet astronomy is a new science, astronomers have only confirmed there are planets around other stars only 31 years ago with the discovery of two rocky planets orbiting a type of neutron star known as a pulsar in 1992.

It has been an amazing ride since then. As of today, we’ve discovered over 5,000 extrasolar planets of all shapes and sizes.

Whenever humans start cataloging and classifying things, whether it be plants, animals, stars or exoplanets, we naturally become interested in the extrema: what is the smallest thing found, what’s the largest? So, in the spirit of being human, let’s take a look at the largest exoplanet found so far.

According to the NASA exoplanet archive, HAT-P-67 b holds the distinction of being the largest exoplanet in terms of size. With exoplanet classification, one always needs to be careful - especially when it comes to size and mass - because there is a fuzzy boundary with large planets where if they become too large, they are considered a brown dwarf star. This is a body that almost made it to star status but doesn’t have enough mass or material for nuclear fusion to take place.

HAT-P-67-b seems to fit the bill. It is 1,200 light years from Earth and has a radius that is a little over twice that of Jupiters’. It orbits its host star very closely, only 10 million kilometers away and has a year that is almost 5 days long. The star itself, HAT-P-67, is an F-type star that is one and a half times the mass of the Sun and about two and a half times larger.

Planets like this are relatively easy to find using the transit method. Large planets that eclipse their stars many times a month can be spotted using space telescopes like TESS because they are looking at an area of the sky for a limited amount of time and they can see many transits during that time.

In contrast, it’s very hard to see a long period planet, one that takes many hundreds of days to transit its star. A planet like Earth, for example, would only make one transit in 365 days to a distant observer so they would need to stare out at our Sun for a long time, especially if they wanted multiple transits.

But HAT-P-67-b was easy to spot. It was discovered in 2017 and since it transits so often confirmation was relatively quick. Because of its low density, astronomers consider this planet a Hot-Saturn, with a mass about half that of Jupiters.

As far as habitability goes, unless there are some moons orbiting this planet getting enough warmth to keep water liquid on its surface, there probably isn’t any life here.