(SciShow Space Intro)

Hank: In 2016, two Cal-Tech astronomers proposed that our solar system is home to nine planets and no, they did not reclassify Pluto.  Instead, they found evidence for a hypothetical Neptune-sized planet, at least 40 billion kilometers from the Sun, far enough away that it would take 15,000 years for it to complete one orbit.  In the last three years, this Planet 9 hypothesis has continued to gain support, but last week, two other scientists proposed that this object could be something a little more exotic.  Instead of a planet out there, they say it could be a black hole, but like a tiny black hole, like the size of your fist, like a chihuahua's head.  This paper hasn't gone through peer review yet, but it's making some big claims and one way or another, it could teach us more about what's lurking in our solar system.

This whole Planet 9 thing came about after researchers noticed something strange about the outskirts of the solar system.  There's a lot of small, rocky and icy bodies out there, and many of them have weird orbits.  One's so weird that they seem to be affected by the gravitational pull of some larger, unknown body, something between 5 and 15 times the mass of Earth.  If there's something out there, it's probably a planet, but technically, any object with the right amount of mass would do, including a special kind of black hole.  

Normally, black holes form from massive stars, so they're millions of times heavier than the Earth, but there's a hypothetical kind of black hole that could be much lighter, like 5 to 15 times the mass of our planet.  They're called primordial black holes, and they may have been created shortly after the universe began.  At that time, everything in existence was packed close together, and as the idea goes, primordial black holes formed when extra dense pockets of matter collapsed in on themselves.  According to this new hypothesis, an object like this could have been captured by the Sun's gravity, and it would easily explain all the weird orbits we've seen past Neptune.

Now since black holes are so dense, this thing would be small, only about 9cm across, but it might still be easier to spot than a distant planet, at least if you don't rely on visible light.  Scientists believe that primordial black holes would be surrounded by halos of dark matter--this is a type of matter that we can't directly detect, but that most evidence suggests is out there.  The authors of this paper argue that occasionally that dark matter around the black hole could interact with similar particles and turn into gamma radiation.  Lucky for us, we have telescopes that can pick that up, so theoretically, if we started seeing gamma ray flashes out past Neptune, it could be a sign that we have a local black hole.

This hypothesis is definitely in need of more evidence, but even if it doesn't pan out, searching for a primordial black hole wouldn't be useless.  It would likely allow us to learn more about dark matter, primordial black holes, and the flashes of gamma rays that we've already detected, so one way or another, it seems like a possibility worth investigating.

In other black hole news, 'cause it's that kind of week, a handful of telescopes has detected something super hardcore, a black hole 375,000,000 light-years away ripping apart a star with the power of gravity.  The results were published last week in The Astrophysical Journal.  The discovery itself happened back in January, and the first instrument to notice something going on was NASA's TESS.  TESS has been orbiting Earth for a little over a year now, and it stares at one large section of sky for several weeks at a time.  Its main goal is to find planets beyond our solar system, but because it's just floating around out there with its proverbial eyes open, it's bound to observe other phenomena, too, and that's what happened last winter.

In January, the telescope picked up an increase in brightness coming from a distant star.  Then, several days later, less sensitive instruments on the ground noticed the same thing.  The event came to be called ASASSN-19bt, after the first project to give us data about it, because even though TESS technically saw it first, it only sends data to Earth every two weeks, so the other team got the naming rights.  This brightening turned out to be the early stages of a tidal disruption event, or TDE, which is a scientific way of saying that a star is getting absolutely wrecked by a black hole.  The murderous culprit sits at the center of a galaxy called--okay, don't make me say that.  I don't know how to say that.  Look, you're never gonna visit this thing.  The bigger point is that this black hole seems to be about 6,000,000 times more massive than the Sun, that's 50% more massive than the supermassive black hole at the center of the milky way.  The more mass an object has, the more of a gravitational pull it has on the stuff around it, so when a star wandered too close to this black hole, things got messy.  

The difference in gravity between one side of the star and the other became so great that it overcame the forces holding the star together.  In other words, the black hole ripped the star apart.  Some of the star's gas and plasma likely escaped into the void of space, but the rest tumbled down toward the black hole, creating a swirling disc and a large flare of radiation that we can see from Earth.  

Tidal disruption events are super rare and scientists have only captured about 40 of them so far.  That means that each new observation can teach us something.  In this case, observing the event early on allowed researchers to chart the extreme drop in the star's temperature that happened within the first few days.  It went from 40,000 degrees Celsius to only 20,000.  Something like this was in our prediction models, but now we have actual evidence.  

Scientists will continue to study this event with both TESS and other instruments and ultimately their data will help them develop better models for how TDEs happen.  From tiny hypothetical objects to monsters that rip apart stars, there's a lot of news about black holes this week, but if these papers show us anything, it's that there's always more to learn.

Thanks for watching this episode of SciShow Space News.  Before you go, I have an update for you.  Every month, we release a new space-themed pin, and October's pin is officially available.  It's of Sputnik, humanity's first artificial satellite, and it's very very good and shiny and fun, and you can get it only during the month of October, so if you're interested, check out the link in the description or the merch shelf below the video.

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