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Scientists have worked out how likely it is that distant planets can see earth, and we are learning new ways to study the magnetic fields of galaxies.

Host: Hank Green
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Sources:
https://academic.oup.com/mnras/article-abstract/doi/10.1093/mnras/stx2077/4082231/Transit-Visibility-Zones-of-the-Solar-System
https://exoplanets.nasa.gov/interactable/11/
http://exoplanet.eu/catalog/
http://adsabs.harvard.edu/doi/10.1086/166608
http://www.nature.com/nature/journal/v355/n6356/abs/355145a0.html
https://futurism.com/the-first-exoplanet-was-discovered-25-years-ago-today/
https://www.nature.com/articles/s41550-017-0218-x
https://www.cfa.harvard.edu/~mengsu/My_Research/Entries/2011/5/1_Galactic_Magnetic_Field.html
http://www.scholarpedia.org/article/Galactic_magnetic_fields
https://www.nature.com/nature/journal/v362/n6421/abs/362610a0.html
https://arxiv.org/pdf/1604.00028.pdf
https://arxiv.org/pdf/1302.5663.pdf
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http://iopscience.iop.org/article/10.1088/0004-637X/706/1/L155/meta
http://www.wolframalpha.com/input/?i=redshift+z%3D1.019&rawformassumption=%22FSelect%22+-%3E+%7B%7B%22LookbackTimeFromRedshift%22%7D%7D
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Images:
https://www.nasa.gov/image-feature/cassini-earth-and-saturn-the-day-earth-smiled
https://exoplanets.nasa.gov/interactable/11/
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https://www.nasa.gov/content/satellite-view-of-the-americas-on-earth-day
https://www.nasa.gov/image-feature/goddard/2017/hubble-displays-a-dwarf-spiral-galaxy
https://www.nasa.gov/image-feature/jpl/spitzer/hubble/pia19837/what-feeds-the-beast-in-a-galaxy-cluster
https://www.cosmos.esa.int/documents/387566/425793/2015_353GHz_B-field/6965c458-b93b-4d08-8395-899157011727?t=1423086190400
Scientists are always keeping an eye out for extraterrestrial life, and finding friendly, intelligent life on another world?

Kind of a dream. But if we ever want a long-distance relationship with aliens, finding them is only half the battle.

They also have to find us. And according to a paper published online last month in the Monthly Notices of the Royal Astronomical Society, that might not be so easy. The researchers based their calculations on what someone on another planet would see if they tried to detect planets in our solar system using the main technique we use to detect planets in other star systems.

We’ve found more than three and a half thousand exoplanets since the first discoveries back in the late 80s and early 90s. Scientists have a few different ways of detecting them, but by far the most successful is the transit method: They watch to see if a star seems to get dimmer periodically, which can mean there’s something regularly passing in front of it. Like a planet.

It’s such a simple technique that we’ve used transits to find three-quarters of the exoplanets we’ve discovered so far, but it has its limits — mainly, we can only use transits to find planets that pass between us and their stars. There are lots of exoplanets out there that we just can’t detect using this method, because their orbits don’t line up right. That limitation would also apply for any aliens using the transit method to look for planets outside of their star systems.

They’d only be able to see us if they were in just the right place to spot Earth passing in front of the Sun as we orbit. So, the paper’s authors used the size and locations of our Solar System’s planets to calculate where you’d be able to see them transit the Sun, and what they found wasn’t too encouraging: From any random point in space, intelligent life would only have about a 2.5% chance of being able to see any of the planets in our solar system transit the Sun. And there’s only about a 0.5% chance they’d be able to see Earth, specifically.

They’d have around a 0.2% chance of being able to see two planets, and a 0.02% of seeing three. Which aren’t great odds. But the authors didn’t just do their calculations and end their paper with a “Forever Alone” meme.

They went through all the known exoplanets and found about 65 in the right place to see one of the planets in our solar system, including 9 that could see Earth. And based on what we know about how often different kinds of exoplanets form, they calculated that there might be around ten nearby, Earth-like planets that we can’t see because they’re at the wrong angle, but where you could see Earth transit the Sun just fine. Of course, we don’t actually know if there’s extraterrestrial life out there, let alone intelligent life that knows how to use the transit method to detect planets around other stars.

But if there is another civilization out there wondering if it’s alone in the universe, at least now we know a little more about whether they’d be able to see us. Meanwhile, we’re also learning more about how galaxies work. There’s still a lot that astronomers don’t quite understand about how galaxies form, and one mystery is how they get their magnetic fields.

But in a paper published this week in Nature Astronomy, researchers reported the best-ever measurements of a distant galaxy’s magnetic field, which are helping chip away at some of those unanswered questions. Magnetic fields are really important for helping galaxies keep in order: they help gravity maintain the galaxy’s overall structure, and they help gas clouds collapse to form stars. It’s pretty hard to study magnetic fields from a distance, though, because they tend to be pretty weak, and we can’t see them directly like we can see light.

The problem is, distant galaxies are exactly the ones we need to study to figure out how today’s magnetic fields came to be. They’re so far away that their light has taken billions of years to get to Earth, so they’re like a window to the early universe. Astronomers usually measure a galaxy’s magnetic field by studying how it affects the light passing through it.

The technique works great for mapping nearby galaxies, and even some distant galaxies with super-strong magnetic fields. But it doesn’t work as well for distant galaxies with plain-old average magnetic fields, and without knowing more about average galaxies, we can’t really understand how most galaxies evolved. So the authors of this new study decided to see how light from a really bright, distant galaxy — about 7.9 billion light-years away — was affected by the magnetic field in a slightly closer, more average galaxy, about 4.6 billion light-years away.

All that bright light made smaller effects from the closer galaxy’s magnetic field a lot easier to see, and because of the way these two galaxies were lined up with Earth, the team was able to measure the magnetic field in the closer galaxy really precisely. That gave them our best picture yet of what galaxies’ magnetic fields looked like in the early universe. For one thing, this galaxy’s magnetic field was about as strong as the fields in today’s galaxies, which tells us that galactic magnetic fields probably haven’t changed too much over the last few billion years.

They also found the most distant evidence yet for one of our best explanations of why galaxies have magnetic fields in the first place — a dynamo, swirling gas and intense cosmic rays that sustain the galaxy’s magnetic field by enhancing the smaller fields from things like stars. So, thanks to a couple of galaxies billions of light-years away, we now know a little bit more about how the universe around us came to be. Thanks for watching this episode of SciShow Space News, and thank you especially to all of our patrons on Patreon who help make this show possible.

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