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We've only found one planet in a globular cluster, where gravitational interactions should usually rip baby planets apart, but that's not all that excites astronomers about PSR 1620-26 b.

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Sources:

https://doi.org/10.1038/332045a0
http://adsabs.harvard.edu/abs/1993ApJ...412L..33T
https://doi.org/10.1126/science.1086326
https://arxiv.org/abs/astro-ph/0309502v1
https://www.nasa.gov/centers/goddard/news/topstory/2003/0709hstssu.html
http://www.aspbooks.org/a/volumes/article_details/?paper_id=1394
http://www.aspbooks.org/publications/398/119.pdf
https://arxiv.org/pdf/1012.0572.pdf
https://arxiv.org/abs/1711.09691
https://arxiv.org/pdf/1410.0052.pdf
https://arxiv.org/pdf/1806.11200.pdf
arXiv:astro-ph/0205087

Images:

https://commons.wikimedia.org/wiki/File:M4_region_(26322094205).jpg
https://en.wikipedia.org/wiki/File:Messier_4_Hubble_WikiSky.jpg
https://en.wikipedia.org/wiki/File:The_globular_star_cluster_Messier_4.jpg
https://en.wikipedia.org/wiki/File:Artist%27s_impression_of_pulsar_planet_B1620-26c.jpg
https://svs.gsfc.nasa.gov/10861
https://commons.wikimedia.org/wiki/File:White_dwarf.jpg
https://en.wikipedia.org/wiki/File:The_evolution_of_the_PSR_B1620-26_system.jpg
https://nasaviz.gsfc.nasa.gov/12278
https://nasaviz.gsfc.nasa.gov/11541
https://svs.gsfc.nasa.gov/10659
[♪ INTRO].

In 2003, astronomers discovered an exoplanet named PSR 1620-26 b. It doesn’t have a fancy name, and it’s likely just another gas giant, this time, at about two times the mass of Jupiter.

It probably even formed like all other exoplanets out there. But for scientists, this planet is anything but normal. Even fifteen years after its discovery, researchers believe it’s still the oldest confirmed planet we’ve ever found.

Evidence suggests that it’s 12.7 billion years old, barely a billion years younger than the universe itself. And that had a lot to teach us about planetary formation. Although this planet wasn’t discovered until the 2000s, its story, at least in human history, really started in the 1980s.

In 1988, researchers began studying an object in a neighborhood called Messier 4, or M4. It’s a globular cluster, a very dense group of stars, about 5500 light-years away. M4 has more than 100,000 stars, but this time, scientists were investigating a pulsar called PSR 1620-26.

Pulsars are the remnants of explosive supernovas. They emit beams of radiation and spin really quickly, like super fast lighthouses. And as that beam sweeps across Earth, we see the star flash at extremely regular intervals.

That’s how the pulsars got their name. Except, the timing of this pulsar’s flashes wasn’t quite so regular. They were just a little bit off, and that suggested there was something orbiting the star, its gravity tugging the pulsar a little this way and that and affecting those predictable signals.

So astronomers got to work. And after multiple years of observation and a lot of math, they were eventually able to identify not one, but two objects around the pulsar. The first was a white dwarf, which formed out of a mid-sized star like the Sun.

It’s estimated that M4 contains more than 40,000 white dwarfs, so this wasn’t that unusual. The second object, though, was a planet. And that was a lot more unexpected.

See, by 2003, astronomers had started finding exoplanets all over the place, and we’d even found a few orbiting another pulsar. But no one had ever found a planet in a globular cluster, and many scientists weren’t sure it was even possible. Gravitational interactions in places like M4 can rip baby planets apart, and with tons of stuff flying around, there are lots of major impacts.

It’s just not a great place for planets to form, except, this one managed to survive. Today, it’s actually still the only known planet we’ve seen in a globular cluster. Somehow, though, that still wasn’t the strangest thing about finding a planet in M4.

What was even weirder is that this neighborhood is really old, about 12.7 billion years old. According to what we know about cluster formation, that means the stars within this group are equally ancient. And by extension, so are any planets around them.

Even now, we’re still figuring out exactly how planets form, but the general consensus says that they form out of the disk of matter that orbits a young star, as the matter starts glomming together and kind of snowballing. This is called the core accretion model. This process is supposed to happen soon after a star is born, so planets and their host stars have about the same age.

So since most of the stars in M4 are nearly 13 billion years old, it follows that this pulsar planet is, too. And that makes it likely the oldest planet ever discovered. That’s more than just another record, though.

It also has a lot to teach us about the early universe. See, the thing about the core accretion model is that, if you’re going to form a planet by snowballing stuff, you need to have a lot of heavy, diverse elements. Even gas giants are more than your basic hydrogen and helium.

But it’s not like those heavy elements have existed since the Big Bang. Instead, it took millions of years for them to form. They only began to exist as stars were born, fused lighter elements together, and then spit out the heavier products when they died.

And that’s why finding this pulsar planet was such a big deal. It was the first piece of physical evidence announcing that, nearly 13 billion years ago, there were enough heavy elements to start forming planets after all. It confirmed something researchers had been thinking about for years, and it also hinted that there could be other ancient worlds out there, too.

Since 2003, astronomers have found more old planets, but none of them have been able to topple this record. But from what we now know, they should be out there. So we’ll just have to keep looking.

Thanks for watching this episode of SciShow Space! We love scouring the research for the weirdest, most surprising, and most awe-inspiring stuff out there, but we’d love to hear what you want to learn about, too. If you have a suggestion for future SciShow Space videos, go ahead and leave them in the comments, and we’ll look into it.

And as always, you can keep up with the latest videos by going to youtube.com/scishowspace and subscribing. [♪ OUTRO].