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Is it a bird? A plane? A supernova? No! It turned out to be something else!
We've also now studied the weather on a gas giant exoplanet!
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Hank: The record holder for the largest supernova ever recorded just lost its title. Earlier this year, we told you about the discovery of what seemed to be the biggest-ever star explosion, based on a signal coming from a distant galaxy. We also told you there was a slight possibility that this wasn’t a supernova at all, and that the signal was coming from something else.

And in a paper published this week in the very first issue of the journal Nature Astronomy, a team of researchers announced that in the end, it probably wasn’t a supernova. It was just a really bright bird that flew in front of a telescope... it was not that. Based on the data they’ve collected since the record-breaking supernova was announced, they’ve concluded that the signal is coming from a different vicious, star destroying process — what’s known as a tidal disruption event, where a star gets ripped to shreds by a supermassive black hole.

In this case, that star was about the size of our sun. As this unlucky star passed near the black hole, the side of the star closest to it would have felt a stronger pull from the black hole’s gravity. Eventually, the difference between the gravitational pull on each side of the star would have become stronger than the gravity holding the star together. And that is when the star would have been violently torn apart.

If that’s what happened in this case, it would release a staggering amount of energy, causing the bright flash that was originally thought to be a supernova. And there are a few lines of evidence that point to this signal coming from a star being ripped apart by a black hole, rather than from a supernova.

For one thing, the event took place at the center of the star’s galaxy, exactly where a supermassive black hole would live, and where you’d be less likely to find the kind of star that would explode in a supernova. The flash, which the team has now observed for the better part of a year, has also brightened and dimmed in ways you wouldn’t expect to see during a supernova.

All this new data is also helping uncover the nature of the objects involved. For example, the paper argues that this black hole must be more than a hundred times more massive than the one at the center of the Milky Way, and spinning really fast. So, that record-breaking supernova probably never happened. It was just a star being torn apart by a huge, spinning supermassive black hole. Still pretty cool!

Rotation also played a big role in another discovery announced this week in Nature Astronomy. For the first time, astronomers have been able to study the weather on a gas giant planet outside of our solar system.

As we’ve discovered more and more planets around other stars, understanding what those worlds are like has become increasingly important and increasingly possible. And one way to learn more about a planet is by studying its atmosphere. But that’s a big challenge, because when you’re analyzing the light coming from a star system, it’s hard to separate details about a planet’s atmosphere from all the other information you’re getting.

But the researchers were able to learn a lot about the planet HAT-P-7 b’s atmosphere, including what the weather might be like. The planet is an example of a hot Jupiter, a class of planets at least as big as Jupiter that orbit closer to their star than Mercury does to the Sun. The Kepler Space Telescope spent four years observing this world by repeatedly measuring the brightness of its star.

When HAT-P-7 b passes in front of the star, it blocks a small-but-detectable fraction of its light. But even when it’s not between us and the star, it still changes the star’s brightness, because it reflects some of the star’s light back to us and emits some of its own. If the weather on the planet stayed pretty much the same all the time, its contribution to the star’s light would repeat a nearly exact pattern, orbit after orbit.

But that’s not what we’re seeing here. Instead, Kepler data reveals that the point where HAT-P-7 b shines brightest changes over time — probably because the planet always keeps the same face towards the star, in a configuration known as being tidally locked. The side that’s always facing the star is naturally much hotter than the other and this big imbalance in temperature leads to high-speed winds that circle the planet.

These winds sweep clouds that form on the cooler nightside to the much hotter dayside. And those clouds change the reflectivity of the planet. Since, like on Earth, cloud formation is a somewhat random process, this leads to the variable brightness that Kepler detected.

Future observatories like the James Webb Space Telescope should be able to study this process in more detail, which will reveal much more about the composition and structure of exoplanet atmospheres. In the meantime, we now know what the weather is like on a gas giant outside the Solar System. And based on the forecast, you probably would not want to have a picnic there.

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