This illustration depicts the three giant planets orbiting the Sun-like star Kepler 51 as compared to some of the planets in our solar system. There is no direct imaging. Therefore, the colors of the Kepler 51 planets in this illustration are imaginary. Credits: NASA, ESA, and L. Hustak and J. Olmsted (STScI)

Why are these exoplanets nicknamed “super-puffs” by Scientists?

A rare class of young Worlds that have “cotton candy densities” when compared to their sizes

Faisal Khan
Jan 8 · 3 min read

The Cosmos is filled with weird & strange phenomena — ones that stretch our imagination and challenge our understanding of the universe within the current framework of scientific concepts. I have touched base on a few of these mysteries in my previous articles — Fermi bubbles, Clouds orbiting a galaxy, exploding rocks from an asteroid & missing dark matter among galaxies to name of a few.

Seems like we have to add one more to the growing list of unknowns. Recent data from NASA’s Hubble Space Telescope provides clues about the chemistry of these puffy planets. The Jupiter sized exoplanets are located in the Kepler 51 star system, which was discovered by NASA’s Kepler space telescope in 2012. The surprise came in 2014 when the ultra-low densities of these exoplanets, orbiting a young Sun-like star, were discovered.

However, recent Hubble observations enabled the researchers to determine the mass & size of these planets with more precision. Two factors were considered in measuring the masses of these planets — change in orbital timings caused by the massive gravitational pull of these gas giants among each other combined with the duration when the planets pass in front of their host star (an event called a transit).

While the results for Kepler-51 b verified the previous observations of mass, the Kepler-51 d was found to be slightly less massive (puffier) than the previous measurement. To put things in perspective, these Jupiter sized exoplanets were only a few times massive than the Earth or roughly a hundred times lighter than Jupiter.

The team concluded that these ultra-low densities of Kepler exoplanets could be attributed to the younger stellar age of the system — coming in at only 500 million years old, compared to our 4.6-billion-year-old Sun. Models suggest that these planets formed outside the “snow line” region — an area where icy materials can survive, but they eventually moved closer to the star.

This close proximity to the star would cause their low-density atmospheres to evaporate over the next few billion years and Kepler-51 b would transform into a smaller & hotter version of Neptune, but the distant Kepler-51 d would continue to be a low-density oddball planet albeit shrinking a little.

Apart from this, the NASA research team also looked for traces of water on the planets Kepler-51 b and 51 d. However, they could not find any chemical signatures in the atmospheres of both these planets to confirm this. This endeavor was also partly hindered by clouds of particles high in their atmospheres. How and why the atmospheres of the exoplanets balloon outwards remains unknown.

Not all is lost though. With the James Webb Space Telescope becoming operational in 2021, it would provide much better observations in determining the atmospheric composition of these two planets. The JWST’s increased sensitivity to longer infrared wavelengths of light should be able to pierce the upper cloud layers and give astronomers better insights about the atmospheres of these exoplanets.

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Faisal Khan

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