"Super-Puffs" sounds like a newly launched breakfast cereal, but it's actually a unique and rare nickname for exoplanets. They are as dense as marshmallows, and there are no such planets in our solar system.
This image depicts three giant planets discovered by the sun-like star Kepler-51 and NASA's Kepler Space Telescope between 2012 and 2014. These planets are about the same size as Jupiter, but much less massive than Jupiter. According to the hubble space telescope's latest observations, they are extremely low density, more like foam polystyrene than rock or water. These planets are likely to form far away from the star and then migrate inward to their current locations. Now their expanding hydrogen/helium atmospheres are slowly seeping into space, potentially leaving smaller planets behind. If we look at the sun from Kepler 51 (a distance of about 2600 light-years) along the Orion spiral arm of the Milky Way, we will see the starry sky shown in this picture. But our sun is too weak to be seen in a simulated naked-eye view.
Credits: NASA, ESA, and L. Hustak, J. Olmsted, D. Player and F. Summers (STScI)
New data from NASA's Hubble Space Telescope provides the first clue to the chemistry of the two ultra-soft planets. Kepler-51, an exoplanet system discovered by NASA's Kepler Space Telescope in 2012, has three ultra-soft planets orbiting a young sun-like star. It wasn't until 2014 that scientists determined that the planets were low in density.
A team of researchers has estimated the mass and size of these planets through recent observations, confirming their "soft" nature. Although these planets are only a few times the mass of Earth, the hydrogen helium in them has expanded to almost the size of Jupiter. In other words, these planets may look as big and bulky as Jupiter, but a hundred times lighter in mass.
The reasons for the outward expansion of planetary atmospheres are unclear, making them a major target for atmospheric studies. The team used Hubble to look for evidence of water in the atmospheres of Kepler 51b and 51d planets. As planets pass past stars, Hubble observes infrared rays through the planet's atmosphere, and astronomers speculate on how much light is absorbed to find clear signs of chemical composition in the atmosphere, such as water.
To the team's surprise, neither planet's spectra had any obvious chemical signatures. They attributed this result to clouds of particles in the atmosphere. Jessica Libby-Roberts of the University of Colorado boulder said, "It was completely unexpected. We thought we would see a lot of water molecules, but we didn't. "Unlike clouds made of water on Earth, clouds on these planets may be made up of salt crystals or photochemical haze, as was found on Titan, Saturn's largest moon.
The image above compares the size of the three giant planets around Kepler 51 with those of our solar system. The Kepler Space Telescope detects planets through shadows that form as they pass in front of the host star, without direct imaging. Therefore, the colors of the kepler-51 planets in the figure are fictional, not their real colors.
Credits: NASA, ESA, and L. Hustak and J. Olmsted (STScI)
These clouds gave the team an idea of how Kepler 51b and 51d differ from other low-mass, gas-rich planets outside our solar system. After comparing the spectra of the two planets, the scientists hypothesized that the formation of clouds was related to the temperature of the planet, and the cooler the planet, the more clouds there were.
The research team also explored the possibility that these planets are not actually "ultra-soft" at all. Gravitational pull between planets causes their orbital periods to vary slightly, and planetary masses can be calculated from these time effects. By combining the time changes in a planet's passage from before the star, known as a transit, with the transit observed by the Kepler Space Telescope, the research team more accurately limited the planet's mass and system dynamics. Their results were consistent with previous measurements of 51b, but at the same time, they found that the mass of 51d was a little lighter than previously predicted.
Ultimately, the team concluded that the low density of these planets is partly due to the youngness of the planetary system (only 500 million years), compared to our sun's 4.6 billion years. Models suggest that the planets formed outside the "snow line" of stars , the regions where ice can exist in orbit , before migrating inward.
Now, as planets move closer to their stars, their low-density atmospheres will evaporate into space for billions of years to come. Using models of planetary evolution, the team was able to show that Kepler-51b, the closest star, would one day (billions of years later) resemble the smaller and hotter Neptune, a planet commonly found in the Milky Way. Kepler 51d, farther from the star, will remain a low-density planet after shrinking and losing a small amount of atmosphere. Zach Berta-Thompson of the University of Colorado boulder said: "This planetary system provides a unique laboratory for testing theories of early planetary evolution. ”
The good news is that the task of determining the composition of the atmospheres of these two planets is already on the agenda. The upcoming James Webb Space Telescope is more sensitive to far-infrared light and may be able to capture signals through clouds. Future observations could provide information about the actual composition of these marshmallow planets. Until then, these planets remain a sweet mystery.
original:
https://www.nasa.gov/feature/goddard/2019/cotton-candy-planet-mysteries-unravel-in-new-hubble-observations