Is a bicycle like a motorcycle? Of course, the answer is it is and it isn’t. Saying something is “like” something else presupposes a lot of hidden assumptions. In the category “things with two wheels,” we have a winner. In the category “things that require gasoline,” not so much. We’ve noticed before that news stories about astronomy often talk about “sun-like stars” or “Earth-like planets.” But what does that really mean? [Paul Gilster] had the same questions, if you want to read his opinion about it.
[Paul] mentions that even textbooks can’t agree. He found one that said that Centauri A was “sun-like” while Centauri B was sometimes considered sun-like and other times not. So while Paul was looking at the examples of press releases and trying to make sense of it all, we thought we’d just ask you. What makes a star like our sun? What makes a planet like our planet?
Part of the problem is we don’t really know as much as we would like about other planets and their stars. We know more than we used to, of course. Still, it would be like wondering if the motorcycle was like that distant point of light. Maybe.
This is one of those things that seems deceptively simple until you start thinking about it. Is a planet Earth-like if it is full of water? What if it is totally covered in water? What if there’s no life at all? But life isn’t it, either. Methane-breathing silicon-based life probably doesn’t live on Earth-like planets.
Maybe Justice Potter Stewart was on to something when he said, “I know it when I see it!” Unfortunately, that’s not very scientific.
So what do you think? What’s a sun-like star? What’s an Earth-like planet? Discuss in the comments.
Don’t even get us started on super-earths, whatever they are. We are learning more about our neighbors every day, though.
Yawn. Like nothing new under the sun-like sun.
For me the definition of an Earth-like planet would be the one to harbor a civiliation advanced enough to embrace a goat, see?
Embrace a goatse? 🤔
Gotta colect those rosebuds somehow, though I’m more into edging recently.
The problem lies with using a binary decision point to describe a distance.
All objects and concepts in the language exist in the multidimensional space of descriptive variables. Things like size, weight, and color are specific dimensions within that space, and serve to nail down the coordinates of the concept within the space.
Similarity is the distance between objects within that space. You could show someone a picture and ask “is this a cat or a dog”, and the person might respond “it looks closer to a cat”. This is a verbal cue to what’s going on in the brain – the mechanism of recognition (of cats and dogs) is a sort of distance used by the brain, and the distance between the picture and that person’s mental “image” of a cat is smaller than the distance to their image of a “dog”.
Another way of saying this is that you would need a number of changes to make the image look like a cat, or a larger number of changes to make it look like a dog. Fewer changes make the image “closer” to “cat”.
Descriptive difference is the amount of information you need to go from one object to another.
(This is similar to the Kullback-Liebler distance, which you can find on Wikipedia.)
Descriptive distance is not an absolute measure, it can only be used in comparison to other descriptive distances. Also, it’s not a distance in the mathematical sense (of distance), because the descriptive distance from A to B may be different than the distance from B to A.
Still, thinking about description as a “distance” makes for a good mental model.
Now about the article…
All stars are like the sun, in the sense that any star is more like our sun that a bicycle is like our sun. The descriptive distance between a red giant and our sun is smaller than the distance from a bicycle to our sun – just make the red giant smaller and yellower and it matches our sun exactly, while the bicycle requires much more cognitive change.
To answer the question posed in the article we need to first place restrictions on the various dimensions within the cognitive space. We might say that “sun-like” is firstly a star, mass must be between 0.9 and 1.1 of sol’s mass, specific color range, and maybe in a non-dusty environment (the purpose of the designation being “life supporting”, we might say that a star within a dusty nebula isn’t life supporting). Or impose similar bounds.
Once we do that we have to label the restrictions as specific to the usage. We would say “sun like, in the astronomical sense” to indicate that the restrictions are important to the discussion.
Because, of course, there are ways in which just about anything is similar to our sun. It’s made of matter, for example.
Just like bicycles.
Worth reading, thank you.
We can’t even agree on what is a planet, in our own solar system.
A large round thing that orbits a star. Like Pluto. That’s my favorite planet.
Obligatory xkcd
https://xkcd.com/3063/
What’s the domain of comparison? Our sun and Alpha Cen A are pretty similar. Compared to those two, Tau Ceti is kinda dim, but compared to Procyon, all three are pretty similar. And compared to Spica or Canopus, all those main-sequence dwarfs are pretty similar.
And then, of course, there’s VY Canis Majoris.
“I know it when I see it!” Unfortunately, that’s not very scientific.
Well you know … really that is the best description because ‘words’ are not just True or False. Words have a lot of grey area as well. Dare I say the word ‘fuzzy’?
A sun like star is a star that’s located in the Solar System, that emits ~1200W/m² at 1 A.U centered on ~400nm, with a mass of one sun. It’s must be 4.6 billion years old. To my limited knowledge, there’s only one of them.
let’s not forget Jupiter that could have been a binary “sun” if it was somewhat bigger at the start.
Maybe “Sun-sized” and “Earth-sized” should be used instead. “Sun-sized” avoids the “Sun-like” attributes, like seven orbiting planets and one orbiting former planet. “Earth-sized” avoids the “Earth-like” attributes of a N2-O2 atmosphere, 5 (or 7?) oceans, an orbiting Moon-like moon, and seven continents (in one of which there is a President Trump).
I think the only way to decide that is to describe why you want to know if they are similar. If I want to go fast and feel the wind, a bike and a motorcycle are similar. If I want to go a long distance with little effort, a bike is not an option.
So why do you care if something is sun-like? I assume it has to do with the potential for life-supporting planets nearby. So then its pretty easy; decide what features make the sun useful for supporting life-supporting planets, and determine if your candidate star has those features.
Even “supporting life” is a vague measurement. There is a limited zone of habitability around the sun – Venus is basically a hellscape and Mars is dead as can be. But even then, what’s the definition of supporting life? There are thermal vents at the bottom of the ocean with heats that would literally kill us that somehow have entire ecosystems thriving around them. Food for thought.
The amount of likeness to the sun that any given star displays is proportional to how badly the guy studying said star needs a grant this year
Aren’t sun-like stars the ones with at least one class-M planet? That’s what it seems like based on those historical star mission logs.
Ahhh yes, the historical records!