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Some tidally-locked exoplanets might actually be more habitable than astronomers initially thought, and we have some ideas about how Peter Pan disks can last so much longer than other protoplanetary disks.

Hosted by: Hank Green

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Sources:
http://dx.doi.org/10.1093/mnrasl/slaa098
https://www.eurekalert.org/pub_releases/2020-06/qmuo-adh060920.php
https://hea-www.cfa.harvard.edu/lifeandthecosmos/wkshop/sep2012/present/m_guarcello.pdf
https://arxiv.org/pdf/2001.05030.pdf

http://dx.doi.org/10.1038/s41467-020-16543-8
https://www.eurekalert.org/pub_releases/2020-06/uoe-poa060820.php
https://climate.nasa.gov/news/807/dusts-warming-counters-half-of-its-cooling-effect/
https://daily.jstor.org/can-single-volcano-cool-earth/

Images:
https://svs.gsfc.nasa.gov/11693
https://nasaviz.gsfc.nasa.gov/12278
https://svs.gsfc.nasa.gov/11541
https://hubblesite.org/contents/media/images/2007/16/3800-Image.html?Type=08-wall-murals
https://en.wikipedia.org/wiki/File:Peter_Pan_disk.png
https://svs.gsfc.nasa.gov/11477
https://en.wikipedia.org/wiki/File:Gliese_581_-_2010.jpg
https://svs.gsfc.nasa.gov/13496
https://en.wikipedia.org/wiki/File:TRAPPIST-1d_Artist%27s_Impression.png
https://en.wikipedia.org/wiki/File:TRAPPIST-1f_Artist%27s_Impression.png
https://svs.gsfc.nasa.gov/11751
https://svs.gsfc.nasa.gov/12983
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Go to ridge.com/scishow and use promo code “scishow” to get 10% off your next order. [♪ INTRO]. The universe is an awfully big place, so astronomers rarely get as close to things in space as they’d like.

One way around this problem is to test ideas using computer simulations, which are informed by our understanding of nature. And last week, two studies came out that took this approach to try and better understand planets outside our solar system and how they form. The first was published in the Monthly Notices of the Royal Astronomical Society: Letters, and it explores so-called Peter Pan disks.

These objects are unfortunately not home to a hook-wielding space pirate; instead, they are protoplanetary disks that seemingly never grow up. Protoplanetary disks are disks of gas and dust that surround young stars. And through a process that scientists still don’t totally understand, out of these disks grow the planets, moons, asteroids, and comets that make up a solar system.

This is a fast process, as far as things in space are concerned. Most protoplanetary disks last only a few million years, and virtually all of them have disappeared after 10 million years. But in 2016, citizen scientists helped astronomers identify a disk around a star that appears to be 45 million years old, and since then, several more of these “Peter Pan disks” have been found.

This new study tries to explain how these objects could last so much longer than others. Using a computer model, they simulated star-forming regions with a range of conditions to figure out which attributes lead to Peter Pan disks. And the results suggest that two factors are especially important.

The first is what’s going on around a forming disk. Most stars form in large clumps that often contain 100,000 stars or more packed close together. But those environments are also dense, with lots of stellar radiation that can cause protoplanetary disks to evaporate away.

So Peter Pan disks need to be loners to survive. The disks also need to start off being extremely large. Which kind of makes sense.

A disk that starts off with a lot of material can afford to lose more than usual. But this also helps explain a curious feature of Peter Pan disks, which is that so far, they’ve only been found around low-mass stars. Now, this could be a sampling error, but the authors suggest it could also be that high-mass stars are just more likely to form in those dense groups of stars, where Peter Pan disks are less likely.

As with other modeling studies, astronomers will need to see a lot more. Peter Pan disks to confirm that these ideas hold up, but their existence alone is a reminder of how variable the process of planet formation is. The second study last week was published in Nature Communications, and this one looked at another aspect of planets around small stars: their habitability: that is, whether life as we know it could survive there.

Or technically, it’s where liquid water could stably exist on the surface, which accomplishes approximately the same goal. Many planets around dwarf stars are tidally locked, which means that the same side of the planet always faces the star. As you can imagine, somewhere where it’s either always day or always night doesn’t seem like an ideal place to live.

The side facing the star often heats up dramatically, while the far side can be extremely cold. But in computer simulations run by the authors of this paper, they suggest there might be an antidote: dust. Dust may sound mundane, or even like a bad thing.

But the effects of dust on climate are much more nuanced than you might expect. On Earth, the main role of high-altitude dust, so far as we can tell, seems to be in cooling the planet. As light reaches the Earth from the Sun, some of it hits particles of dust and is reflected back into space.

That’s why volcanic eruptions can have a measurable impact on the Earth’s average temperature. But dust doesn’t just cool the planet, it also warms it up. See, planets radiate heat, and some of that heat gets absorbed by dust and is trapped in the atmosphere before it can escape to space.

Overall, though, our current understanding is that dust cools our planet more than it warms it up. But on some tidally-locked exoplanets, the researchers suggest the picture might be more complicated. Their simulations indicate that on the day side of a planet, atmospheric dust cools more than it heats.

But on the night side, the opposite happens. The net effect on the planet is one of moderation. The hot, day side ends up cooler than it would be otherwise, while the cold, night side gets a bit warmer.

So with smaller temperature extremes, some of these planets might actually be more habitable than astronomers would initially think. The big picture here is that the universe is, as usual, more complicated than we like to assume. If we rely on broad statements like “protoplanetary disks are short-lived” or “tidally-locked worlds aren’t habitable,” we’ll be missing the nuance we might really need to understand what’s really goin’ on.

Fortunately, computer simulations can help scientists identify their blind spots by testing their ideas with scenarios we haven’t seen in nature. Which, hopefully, will result in faster progress and more new discoveries. Thanks for watching this episode of SciShow Space News!

This week’s episode is brought to you by the folks at The Ridge, makers of The Ridge Wallet. They’re making a thin, light wallet that’s designed to fit in your pocket easily, and to carry up to a dozen cards plus cash without bulging. They also come with a lifetime warranty.

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And if you want a wallet, you can get 10% off and free worldwide shipping by using the promo code “SCISHOW.” [♪ OUTRO].