Exoplanet Misconceptions

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Exoplanet Misconceptions

Post #1by SevenSpheres » 08.06.2021, 18:32

There seem to be several commonly held misconceptions about exoplanets (and related topics). This thread explains why they're wrong. pedro_jg suggested making a topic like this on Discord about a month ago; well, here it is. Work in progress, more entries will be added.



HD 100546 b is the largest known exoplanet, at 6.9 Jupiter radii.

The NASA Exoplanet Archive does list a radius of 6.9 Jupiter radii for this planet or brown dwarf, but if you look at the original discovery paper, you'll see that this is the radius of the directly imaged emission area around the planet, not necessarily of the planet itself. Applying Occam's Razor, the size of the emission area is far more likely to be caused by a circumplanetary disk, rather than a planet or brown dwarf almost as big as the Sun (which as far as we know is impossible); this is also stated in the paper.

So what's the real largest exoplanet? It could be HAT-P-67b. This planet has a reliably measured radius of 2.085 times that of Jupiter from the transit method; it's a hot Jupiter orbiting very close to its star, causing its radius to be inflated from the heat.

Some text on planetary habitability will be added here in the future.
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Misconceptions about color

Post #2by SevenSpheres » 08.06.2021, 18:33

G-type stars (like the Sun) visually appear yellow.

This isn't exactly about exoplanets, but it's such a common misconception that it has to be addressed here. The Sun, and other G-type stars, are actually white. This should be obvious if you think about it, since sunlight is white. Additionally, images of the Sun from space appear white (example). Some images of the Sun do look yellow or orange, but these are false-color images taken in non-visible wavelengths. The Sun looks yellow, orange, or red at sunrise and sunset due to atmospheric scattering.

The colors of stars follow the blackbody spectrum. This website is old but has some good information on the blackbody spectrum and star colors; there are tables of star colors by spectral type and temperature. They derive a slightly off-white color for the Sun. Our own Askaniy's True Color Tools calculate a virtually pure white color from solar spectra. You'll also notice from these sources that red dwarf stars are more orange than red.

The term "yellow dwarf stars" commonly used to refer to G-type stars comes from the historical use of the well-studied star Vega as a white point. G-type stars are yellow relative to Vega. Vega, however, visually appears blue.

512px-Color_temperature_black_body_800-12200K.svg.png
The blackbody spectrum. Credit to Bhutajata on Wikimedia.
512px-Color_temperature_black_body_800-12200K.svg.png (4.79 KiB) Viewed 2114 times

suncolor.png
The color of the Sun calculated from spectra using Askaniy's true color tools.


TrES-2b and WASP-12b would visually appear pitch black, due to their low albedos.

These planets do indeed have very low albedos (1.36% and <6.4%, respectively), and as such have been described as "pitch black" by the media. However, both of these planets are hot Jupiters, orbiting very close to their stars and receiving large amounts of light. Reflecting even such a low percentage of the light they receive would still make them quite bright! Even at Earth-like illumination, these planets wouldn't be pitch black - here is one Discord member's approximation of the true brightness of WASP-12b. (Eric Nelson notes that an older paper gives a possible albedo as low as 0.04% for TrES-2b. Still, even if this were the case, the planet would not appear pitch black.)

59 Virginis b (aka Gliese 504 b) would visually appear magenta.

Well, it could appear magenta, but there's no evidence that it does. 59 Virginis b has been imaged in the infrared, but there are no visible-light spectra. This confused a lot of people on Discord at one point; we thought its visible-light color had actually been determined but couldn't find a source. The idea that 59 Virginis b is magenta comes from the NASA press release, which speculates on the appearance of the planet:
"If we could travel to this giant planet, we would see a world still glowing from the heat of its formation with a color reminiscent of a dark cherry blossom, a dull magenta," said Michael McElwain, a member of the discovery team at NASA's Goddard Space Flight Center in Greenbelt, Md. "Our near-infrared camera reveals that its color is much more blue than other imaged planets, which may indicate that its atmosphere has fewer clouds."
What's correct here is that 59 Virginis b is cooler and "bluer" in the infrared than other directly imaged planets (as stated in the discovery paper as well). The claim that the planet glows magenta from heat is wrong though, since the planet has a temperature below the Draper point, so it actually wouldn't glow in visible light. Magenta also isn't a blackbody color; however, it seems that some brown dwarfs actually would appear violet!

Pink.png
Pink is a rare color in the universe. Credit to Askaniy.

So what exoplanets do we actually know the visible-light color of? There are several planets (mostly hot Jupiters) with visible-light spectra allowing (approximate) colors to be determined; fyr02 has made a list here. The two exoplanets with the best-determined colors are HD 189733 b and Upsilon Andromedae b, which are both blue.

On a similar note, V838 Monocerotis is sometimes claimed to be pink based on a false-color Hubble image. It's not pink in visible light any more than, say, these stars are.
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Post #3by SevenSpheres » 08.06.2021, 18:33

reserved
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Post #4by trappistplanets » 08.06.2021, 20:12

https://www.youtube.com/watch?v=9QmEKGEC2B0
huge pulsar properties misconceptions

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Post #5by Eric Nelson » 08.06.2021, 21:20

Well we need to keep improving technology to see what such stuff out there really looks like and that's pretty much a whole new level of telescopic exploration.
Another misconception of exoplanets is about highly "habitable" planets orbiting red dwarfs when they're tidally locked and numerous red dwarfs emit frequent and extremely powerful flares which (combined with the planets being much closer to their red stars than our Earth is to Sol) would expose such terrestrial planets to lethal doses of stellar radiation, making them more like Mercury or Mars rather than Earth.
Among the most likely candidates for such a planet may be Kepler-442 b as well as the unconfirmed Kepler-452 b and even TAU Ceti e as they orbit host stars that are more similar to ours than to red dwarfs and are more likely to have rotation periods that aren't tidally locked, even though if there aren't any moons and any such life on those would be very different from us for example (plus greater gravity on those would mean complex living things there could be plants or coral reefs and creatures on those would peak at sizes and weights comparable to domestic pets).
Yet the most "habitable" planets in the TRAPPIST-1 system (which hosts a modestly active red dwarf) are likely very limited in life and likely to host anywhere from bacteria such as algae or up to small deepwater marine creatures especially toward the terminators (the lines between day and night) if any complex life exists there and TOI-700 d (which orbits another moderate red dwarf) also certainly seems less likely to be habitable than Kepler 442 b or 452 b (if that planet even exists) which we still don't quite know the conditions there are like and TAU Ceti e isn't likely flawless either..
Yet the news media gets all excited about terrestrial exoplanet reports.
We know a very limited amount of what's beyond us and even our technology's far from perfect.
Our current best option pending is the James Webb Space Telescope.
We'll wait and see what it all turns out to be.

Added after 46 minutes 48 seconds:
SevenSpheres wrote:TrES-2b and WASP-12b would visually appear pitch black, due to their low albedos.

These planets do indeed have very low albedos (1.36% and <6.4%, respectively),
Now TRES-2 Ab's albedo was said to be anywhere from the 0.0136 (1.36%) value down to as low as 0.0004 (0.04% based on the best fit models from measurements).
Therefore TRES-2 Ab and WASP-12 Ab are definitely distinguishable from one another in lighting (not just their shapes) even if both are visible.

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Post #6by SevenSpheres » 08.06.2021, 22:49

Eric Nelson wrote:Another misconception of exoplanets is about highly "habitable" planets orbiting red dwarfs [...]

I will be adding to the OP including some text on planetary habitability.

Eric Nelson wrote:Now TRES-2 Ab's albedo was said to be anywhere from the 0.0136 (1.36%) value down to as low as 0.0004 (0.04% based on the best fit models from measurements).

What is your source for this? It's not within the margin of error of the sourced 1.36%+0.22-0.33 value.
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Post #7by Eric Nelson » 08.06.2021, 23:10

Source is this: https://web.archive.org/web/20120317203801/http://www.astro.princeton.edu/~dsp/PrincetonSite/Home_files/darkest_world.pdf
It states even with a commonly understandable albedo of 0.0136 (1.36%), the best fit model measurement shows an albedo of 0.0004 (0.04%).

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Post #8by fyr02 » 09.06.2021, 02:48

Eric Nelson wrote:It states even with a commonly understandable albedo of 0.0136 (1.36%), the best fit model measurement shows an albedo of 0.0004 (0.04%)
This source is from 2011. SevenSpheres's source is from 2012 and is newer.
The most recent source that I can find is from Schwartz and Cohen (2015), which gives an albedo of 0.007.

As for WASP-12 b the albedo is dependent on the wavelength (the color of WASP-12 b is brown) but the average albedo seems to be ~0.04.

Again, due to the strong illumination both planets would be indistinguishable from each other as an albedo difference of ~0.03 is usually not large enough to strongly affect the apparent lightness of the planet.
Image

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Post #9by Eric Nelson » 09.06.2021, 02:54

Well 0.007 (0.7%) is certainly darker than 0.0136 (1.36%) but not 0.0004 (0.04%) which's still not 100% black in either way or definition because that would have to have a total albedo by all means of 0 (0%) and that takes being infinity away from a white light source (which's impossible since all sources like stars, nebulae, supernovae, hypernovae, galaxies and quasars are finite distances from us and we receive such light from all of them though by only negligible amounts).
And the difference between 2 planets is especially enough if one can see well when there (not to make people with different vision feel bad).

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Post #10by Eric Nelson » 10.06.2021, 03:06

SevenSpheres wrote:I will be adding to the OP including some text on planetary habitability.
Things to know about:

Planets must orbit stars at the right distance in order to support life.

Each star of any category is different, including red, orange, yellow and such stars.

Not to mention some stars are more active and some are calmer.
The most likely planets to support life are not only those orbiting the right distance from stars, but those orbiting stars behaving more smoothly.

Planets orbiting the habitable zones of calm red dwarfs could host some microbial life throughout and/or marine or certain plant life (especially near the terminators) while planets orbiting the "habitable zones" of much more active red dwarfs (which emit super powerful and frequent flares) would be more similar to Mercury or Mars and planets orbiting the habitable zones yellow dwarfs may host a wider variety of life (though likely still very different from what we're familiar with).

Plus a planet would also be more habitable if there was a large moon which would provide a more stable axis (unless the axis were already stable in the longterm without the aid of a large moon) and give it beneficial tides, not to mention such a planet also needs some geological activity (though not too extreme of it especially so frequently) for it to have life.

Yet a terrestrial planet orbiting a G-star that's also orbited by a gas giant or more would also be a key (especially if the system has an asteroid belt) as gas giants are massive enough to feed on nearby asteroids, making deadly collisions on such a potentially habitable world more rare.

Plus a strong-enough magnetic field would also protect planets from stellar radiation, adding benefits to life (even if at times a flare could cost a power grid assuming life like us could build technology there).

Plus oxygen, CO2 and other suitable atmospheric elements are also important for life and so is enough water.

Yet a planet must obviously have the right mass as well to support life.

If though/if there's life on planets beyond us, it's certainly unlike any life we're familiar with (besides the common and dominant microbes we all know/don't know).

It takes so many things to get a planet just right to be suitable for life in general.
It's more complex than we all realize and the chance of life on each exoplanet varies on so many factors.

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Post #11by trappistplanets » 10.06.2021, 10:57

Eric Nelson wrote:Planets must orbit stars at the right distance in order to support life.
is a potentaly hasbitible moon of a gas planet able to meet all these requierments
(it can still meet the large moon one of the hasbibible moon orbits far enough from the gas planet, it could have a submoon)

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Post #12by fyr02 » 10.06.2021, 19:40

Eric Nelson wrote:Things to know about:
in general planetary habitability is much more complicated than this.
currently its not know what conditions will foster life outside of those on earth.
current research suggests that active m dwarfs may still be able to hold on to some atmosphere but rn m dwarf habitability is still under contention.
Image

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Post #13by jjcatloaf123 » 11.06.2021, 13:39

Have you ever heard of the Rare Earth hypothesis by Peter Ward and Donald Brownlee before? According to Ward and Brownlee, microbial life may be common in the universe, but complex life may actually be rare, both in space and time.

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Post #14by Trolligi 112477 » 12.06.2021, 09:17

There are a lot of objects that the public thinks are planets but are actually brown dwarfs. J1407 b is a good example. It has 20 jupiter masses, but a lot of people consider it a planet, while it is actually a brown dwarf. Oph 11 b and DENIS J0823-4912 b are other examples of this, and anything with a mass greater than 13 jupiter masses should be considered a brown dwarf.
why do I love wolf-rayet stars so much lol

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Post #15by Eric Nelson » 18.06.2021, 05:49

SevenSpheres wrote:The NASA Exoplanet Archive does list a radius of 6.9 Jupiter radii for this planet or brown dwarf, but if you look at the original discovery paper, you'll see that this is the radius of the directly imaged emission area around the planet, not necessarily of the planet itself. Applying Occam's Razor, the size of the emission area is far more likely to be caused by a circumplanetary disk, rather than a planet or brown dwarf almost as big as the Sun (which as far as we know is impossible); this is also stated in the paper.
Well in reality, the true size of HD 100546 b is actually a mystery despite many claiming it to be a whopping 6.9 x Jupiter's radius.
With much better technology in the hopeful future, including these super powerful telescopes on the way like James Webb and/or such, we should hope for a well-defined calculation of how large or small it itself is rather than the size of the disk/emission cloud around it.

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Post #16by Eric Nelson » 19.06.2021, 02:12

Trolligi 112477 wrote:There are a lot of objects that the public thinks are planets but are actually brown dwarfs. J1407 b is a good example. It has 20 jupiter masses, but a lot of people consider it a planet, while it is actually a brown dwarf. Oph 11 b and DENIS J0823-4912 b are other examples of this, and anything with a mass greater than 13 jupiter masses should be considered a brown dwarf.
Yes, and its hard to visually distinguish a gas giant from a brown dwarf (low mass especially) and there's a reason brown dwarfs are called failed stars.
They don't have enough mass to sustain nuclear fusion but have more mass than planets.


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