https://www.reddit.com/r/askscience/comments/vjsshw/how_do_we_know_what_exoplanets_look_like/
created by bl4ck4nti on 24/06/2022 at 16:47 UTC
316 upvotes, 8 top-level comments (showing 8)
If the planets are hundreds and thousands of light-years away, how do we know what they look like and their characteristics? Also because of how long it takes for the light to reach us, is there a possibility that we are looking at a planet that may not exist in present time?
Comment by UmbralRaptor at 24/06/2022 at 17:17 UTC
177 upvotes, 1 direct replies
By in large we don't know what they look like, just an incomplete combination of the orbit, mass, and radius. For a limited number of planets, we have some idea of the atmospheric composition and/or cloud cover. Are you thinking of artist's representations?
Comment by WarOnTime at 24/06/2022 at 17:28 UTC
48 upvotes, 4 direct replies
There’s a lot we can glean from the electromagnetic spectrum of exoplanets. Due to fortunate chance alignments of their orbits with our line of sight, some of these planets alternatively pass in front of and behind their parent star as seen from Earth. These transiting exoplanets provide a unique opportunity to analyze the atmospheres of these distant worlds. The planet's atmospheric constituents can be revealed by analyzing the characteristic absorption lines they imprint in the spectrum of the star when the star's light passes through the planet's atmosphere during a primary transit.
Comment by PlaidBastard at 24/06/2022 at 20:47 UTC
8 upvotes, 1 direct replies
Extremely educated guessing.
The color of a star, being EXTREMELY overly broad, tells us how big it is, and how bright it is with that information tells us how far away it is. Ignoring a lot of exceptions, you basically measure how bright it is through different colored filters, then do some math and you know its mass.
How much the star (of known mass!) wobbles says how heavy the planet is and how far out it's orbiting.
If it passes in front of the star, how long it dims the light (and how often) tells us how wide the planet is and gives us better data on its actual diameter.
Size, mass, and orbital distance tells us its likely composition, surface temperatures, the elements stable in its part of the nebula that its star system formed from, etc.
We have a pretty good idea of what X amount of iron, Y amount of silicon, Z amount of oxygen, etc. will make, chemically, if you pile them up. We call that 'geochemistry.' If a planet is about as dense as Earth, and close to its star, it's probably mostly silicate and metallic materials. We can do the math to know what those proportions are from its density.
If it's more like the density of water, and it's a really big planet, it's probably an ice or gas giant like Jupiter or Neptune, and we can guess which ices/gases it's made of from the light spectra that shine through its atmosphere when it passes in front of its star...
Sorry I can't go into more detail here on my lunch break, but I hope that all helps!
Comment by [deleted] at 24/06/2022 at 19:08 UTC
5 upvotes, 0 direct replies
[removed]
Comment by toocuteforthisshit at 24/06/2022 at 19:00 UTC
2 upvotes, 0 direct replies
we don’t really know what they look like but we can gain a pretty substantial understanding of composition based on their spectra! we can deduce their size from transits in front of their parent star as well based on how much the star light wobbles or is obstructed in our perspective!
Comment by CaptainSur at 24/06/2022 at 20:56 UTC
2 upvotes, 1 direct replies
You received a couple of excellent answers about determination of some planetary aspects by spectrum observation.
What I would add to that is yes, it is remotely possible we might be looking at a planet that does not exist although for most of the exoplanets cataloged they are "in close proximity" and as long as their stars were stable there is no reason the planet should have suddenly been extinguished from a current form of existence. Many are close enough that over decades of time we will be able to distinguish changes. Others may need a century or more of observation.
But separately, the fact is we do see light arriving now from suns and galaxies that are long, long gone. It could be true even for some of the older suns in our galaxy that are typically further away. The type of star also tells us about its stage in life.
In any case if your thinking "am I looking at a sky of objects long gone" the answer is partially yes. We used to joke in our astronomy classes at university that depending on where we were examining we were looking at a "dead sky".
Comment by seanbrockest at 24/06/2022 at 22:43 UTC
2 upvotes, 1 direct replies
You may find this list interesting.
https://en.wikipedia.org/wiki/List%5C_of%5C_directly%5C_imaged%5C_exoplanets[1][2]
1: https://en.wikipedia.org/wiki/List%5C_of%5C_directly%5C_imaged%5C_exoplanets
2: https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets
It's a list of the planets we have directly imaged. Most of the planets we have "confirmed" are from watching the host star, and seeing if the light dims on a regular schedule. If it does, we assume that a planet is regularly passing in front of the star.
There are a handful though, that we have managed to see directly. That list is small, and the planets are usually only a few pixels in an image, but they're there!
Comment by potassiumbones at 25/06/2022 at 01:44 UTC
2 upvotes, 1 direct replies
Spectrographic analysis, basically they measure the amount of specific wavelengths of light to determine chemical composition, atmosphere, etc.
Dr. Clara Sousa-Silva (Dr. Phosphine) is one such researcher, her official title is quantum astrochemist. Look her up, she’s pretty badass.