I enjoy reading and learning about the universe, but one question I have is how astronomers here on Earth can know details about distant planets and stars? Descriptions of a lot of places seem to indicate atmospheric makeup, surface temperatures, and a lot of other details that I would think could only be determined by a visit from a person or probe. Even our closest neighbor planets have only had a few fly-bys and landings. Periodically, there is a news story about a newly discovered planet in a nearby solar system that they say has similar characteristics as Earth. How do they know this?
How can people determine, with any certainty, the makeup of distant worlds with only a view thru a telescope?
Some 19th century astronomer claimed that we could never determine anything about the chemical content of any star. Then spectrography came along. In fact helium was discovered on the sun before it was ever found on earth (the name is derived from the Greek word for sun).
They are certainly trying to use spectrographs to find the chemical composition of some nearby planets, but I don’t think there are any results so far. It would be very hard. With difficulty, they can determined the orbital period and the mass of nearby extra-solar planets and tell if it is large enough to hold water and if it is at the right orbital distance for there to be liquid water (the “goldilocks” or just-right zone). The gold star goes to anyone who discovers O2 on a planet since that is thought to be a sure sign of life. The reason for that is that oxygen is too reactive to last very long and the only way we can imagine it being regenerated is by living organisms (specifically photosynthesis). I might mention that the appearance of oxygen was an ecological catastrophe since it was a deadly poison to most life on earth at the time. But it happened gradually and organisms evolved to tolerate and ultimately use it.
The gravitational effect of the planet is often visible on the star, which allows us to find out its mass, and probable distance. The atmosphere of the planet would affect the star’s light as it passes through it. Astronomers can use a special disk to block out the majority of the light of star and use a spectrometer or similar to measure the spectra affected by the planet itself. The rest is educated guessing, from how we believe planets form and where they occur, from which we can extrapolate how hot they will be, what the majority of their composition is etc…
The planet they say is similar to Earth is similar because it occurs within the “Goldilocks” zone, the small band around a star where liquid water is possible on the surface of the majority of the planet, and because it is “only” five times more massive (so gravity is ~1.7 times stronger at the surface). It’s mainly significant because it’s relatively close to us (16 light years), most other Earth similars are considerably more distant, the only one closer is Tau Ceti e
It’s important to remember that we don’t know all these details with absolute certainty. There’s always going to be some margin of error. And some details, like if the exoplanet even exists, are sometimes wrong.
Thanks for the replies. Ornery Bob, good info at that link.
So, we use what we know about physics and chemistry combined with what we can visibly see and measure thru telescopes to determine the makeup of these far-away places.
When it comes to chemical composition of planets, astronomers only really care about three things - gas, ice, and rock. These have more-or-less known densities, so if you can figure out how big a thing is and what its mass is, you can figure out which of the three it’s made of.
We figure out how big it is by figuring out how much light it blocks from a star as it passed in front of the star. We figure out how massive it is by looking at how its star wobbles as the planet orbits.
Finally, if you watch how fast the star wobbles, you can figure out the planet’s orbital period, which tells you how close it must be to the star. If you know how hot the star is and how far the planet is from the star, then you know how hot the planet is. That’s how you get descriptions like “ice planet” or “rains sulfur” or whatever.
Some of these news items are pure speculation. Like in this recent artist’s rendering of Gliese 832c.
It is unknown whether the planet has an atmosphere, water, oceans, or most of the other details in the made-for-media image. Most all of the details are in the artist’s imagination. But what we get, reported as news, is a super-Earth and a bunch of speculation about, what if?
Scientists can measure the mass of the planet by its effect upon the wobble of the parent star. They can tell from the distance to the star, and the output of the star, how much radiation the planet receives and if it is possibly in the habitable zone. The type of star and the distance of the planet from the star give clues to the composition of the planet. And as noted by Ornery Bob, a lot can be learned from just the light received.
But some or a lot of what you hear in the news is just wishful thinking and plan old imagination.
Indeed. Fortunately, one can sidestep the hype from pop-sci sources and get directly to the source… We report the detection of GJ 832c, a super-Earth orbiting near the inner edge of the habitable zone of GJ 832, an M dwarf previously known to host a Jupiter analog in a nearly-circular 9.4-year orbit. The combination of precise radial-velocity measurements from three telescopes reveals the presence of a planet with a period of 35.68+/-0.03 days and minimum mass (m sin i) of 5.4+/-1.0 Earth masses. GJ 832c moves on a low-eccentricity orbit (e=0.18+/-0.13) towards the inner edge of the habitable zone. However, given the large mass of the planet, it seems likely that it would possess a massive atmosphere, which may well render the planet inhospitable. Indeed, it is perhaps more likely that GJ 832c is a “super-Venus,” featuring significant greenhouse forcing. With an outer giant planet and an interior, potentially rocky planet, the GJ 832 planetary system can be thought of as a miniature version of our own Solar system. [1406.5587] GJ 832c: A super-earth in the habitable zone
(full paper available for download.)
Since it’s the famous quote along such lines, I suspect you’re actually thinking of the opinion expressed by the 19th century French positivist philosopher Auguste Comte.
Having linked to the Arthur C. Clarke use of the quote - holding Comte to shame - I’ll record that I’ve just noticed that Clarke subtly misunderstood what Comte was arguing. Another translation of the passage is here. Which makes it clear that Comte was explicitly referring to planets. Yet Clarke goes on to suggest that Comte wasn’t referring to planets, but only to stars. No doubt a consequence the quote being sufficiently famous that Clarke was himself quoting it at secondhand.
He starts the chapter on Astronomy on p132 by referring to stars and then in the same paragraph he switches to planets without any meaningful transition. He continues to refer to stars and planets and even comets almost interchangeably throughout the discussion.
I have no problem at all with using a fact of indirect inference from a star to refute his argument. I would, however, argue long and loudly against the point of view that claims that Comte was explicitly referring solely to planets at any time.
Except our observations go far beyond what we can visibly see. Detectors can see much more in the radio-wave spectrum, plus there are infrared and ultraviolet telescopes, and x-ray views. What you and I see in the night sky is only a fraction of the information actually available.
To be completely thorough, there is also a small amount of astronomical information to be had from neutrinos and cosmic rays, as well as a trace of information from actual sampling of space material. And in a few years we’ll also be able to detect and recognize gravitational waves, which will give us new sorts of information (potentially a great deal of it).
