When there is no doubt that a claim exists and is widely accepted/believed is it necessary to find an example of a notable counter-claim to question it?
For example, it is widely accepted and believed that other planets exist in our solar system.
Should it be acceptable to ask a question like "Do planets other than earth exist"? without evidence of someone claiming that other planets do not exist?
TL;DR: We should only demand evidence of notability in one direction, and accept genuine disbelief of the claim by the OP as being sufficiently notable in the other.
Underlying your question is an issue about notability which I think has been brewing slowly for a few months, and I welcome the opportunity to thrash it out.
I am going to start off badly - by digressing to a completely different topic.
We've had a few people who have been unimpressed when they have found the Skeptics.SE community is not full of people who, like them, think that the scientific establishment trying to maintain its hegemony by suppressing the true facts of whatever conspiracy theory they follow. They complain that we are just blind follower of scientific orthodoxy.
Yes, we tend to accept much of the scientific consensus, because practising scientists generally take very similar skeptical approaches to evidence and theory.
However, science doesn't know everything. (As one wit put it, if it did, it could stop.) There are wrong scientific theories. There are even wrong scientific consensuses. It is perfectly in line with skepticism and science to challenge the theories, and demand evidence for them. (It isn't, however, acceptable to deny, ignore or cherry-pick the facts when they are presented.)
Often, there are lone voices, declaring that the "Emperor is wearing no clothes". Very occasionally, they are right. An important (albeit challenging) part of science and skepticism is acknowledging when the evidence shows your theories are wrong.
Now, let me get back to notability...
I see at least four reasons for demanding evidence of notability.
If we are going to ask people to volunteer their time doing the research and writing up the result, we want them to be doing something that will be useful for many people. So, we don't want them to be chasing up some whimsical idea that someone had in the pub.
Providing references to the actual claim helps protect us against fighting strawmen - where we rebutt a misunderstood claim that was never actually made.
If we allow random ideas to be proposed for us to research, we are actually encouraging people to spout more speculative nonsense on the Internet, rather than reducing it, which is probably not what many of us would like.
Asking the question-writer to spend just another couple of minutes researching the question before they post, we probably help a lot of people to answer their own questions without us.
Now, typically, we have only demanded notability to apply in one direction.
If someone asks a question that goes:
A TV celebrity says "Celery juice can cure cancer". I don't believe it. Is is true?
it is sufficient for someone to post evidence that the celebrity said it, but that the OP is doubtful. We don't ask the OP to post evidence that anyone is claiming the opposite.
If someone asks a question that goes:
A TV celebrity says "Celery juice is mildly dangerous. It can cause chemical burns if it contacts the skin." I don't know whether to believe it. Is it true?
we have accepted this, too. Not taking a position until the evidence is in, is a creditable response from a skeptic.
However, when someone asks a question that goes:
My science teacher says every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Is it true?
we have seen a different reaction from some people.
This question is directly challenging a well-established part of scientific consensus. The response has been to reject the question's notability, until some evidence is shown that anyone disbelieves this claim (i.e. that the counter-claim is notable.)
This meta-question is basically asking the community if that is the right response.
If we go back to our reasoning for notability, it makes sense to demand evidence that the counter-claim is notable. If challenging well-understood science is just a whimsical idea from the pub - a random notion only held by one person - it is justifiable to not bother with answering it until the OP can show that it is notable to disbelieve the claims.
This is particularly true if the OPs themselves do not express a position - if even the OP agrees with the consensus view, scrabbling around to defend it is wasting everyone's time.
One problem with demanding this notability of the counter-claim is the subjectiveness of the demand: Who decides whether the scientific consensus around gravity, or homeopathy, or acupuncture, or about diet, is well-enough established that challenging it is considered a perverse point-of-view?
Another problem is that it dismisses the role of the maverick, who can see the consensus view is wrong, and has evidence to support it. I think the concept that the maverick with the right evidence could overturn the orthodoxy is an important one to skeptics, and we mustn't write-off the claims of mavericks without considering the evidence.
But finally, I see that the burden-of-proof is on the scientists here. I acknowledge that Skeptics.SE often plays loose with the burden-of-proof, frequently putting it upon the answerer rather than the claimant. However, this is a case where the burden is in the right place, for once. If scientists want to make a positive claim that gravity follows Newton's Laws, they should be able to show evidence to support that.
So far, I have been assuming that the question is asked in good faith.
It doesn't happen often, but there are also recalcitrants who don't want to acknowledge the evidence that supports the science. It's reasonable to not bother to arguing forever with such a person.
On a related note: Normally, we can answer questions by pointing to high-quality articles as evidence. Sometimes, we get people who include a reference to a high-quality article (that follows the scientific consensus) in the question, and claim they are skeptical of it. In such cases, it is reasonable to ask what part they are unhappy with, putting the burden back on them.
Finally, in the cases where it is isn't sufficient to cite one or two papers, but instead the onus is to explain an entire discipline of science in an answer to a question, then it is reasonable to redirect them to do some reading or attend a course.
I think that our notability rules should be clarified slightly.
We should continue to demand that evidence of notability should be provided for a claim that the OP is skeptical about.
If the OP, in good faith, claims that they do not believe a claim, we shouldn't demand that evidence that their view-point is notable, even if it flies in the face of scientific consensus. It is reasonable to (politely) question the reasons for their maverick point-of-view, to establish that good-faith.
As before, if the OP believes in the original claim, then the whole question doesn't belong on Skeptics.SE.
This does not matter if it is a claim or a counter-claim. If no one believes the claim now or it is not in wide circulation, then it should not be allowed.
Allowing anyone to raise a question doubting a claim without providing evidence is not only not productive but is potentially harmful to the site.
It allows for questions such as "Does the moon exist?", "Are there really cane toads in Australia?", "Are elephants really bigger than people"? etc.
Whether or not these questions are asked in good faith or not is irrelevant. We shouldn't want these kinds of questions on the site. I've met adults who genuinely did not accept other planets existed. I would not want to see them questioning such basic facts on this site, not when there are a wealth of resources easily accessible that should be more than satisfactory.
Indeed, we would likely be quoting from such resources in our answers, so what would be the point?
I think allowing questions that doubt a claim without evidence of a counterclaim are harmful for the following reasons:
Being skeptical of the scientific consensus simply does not make sense here.
This is a site for scientific skepticism. If someone is skeptical of a well established scientific consensus, then this is not the right site to visit.
The only answers that are acceptable on this site are going to draw from science papers, the same sources that led to the consensus in the first place. That means that there is no answer we can give that would satisfy a claim.
Questions such as the one asking if atoms exist come across as idle curiosity that could be answered by a Google search. We don't allow list type questions on SE, so asking questions of the type "What is the evidence for X" should not be on topic.
If someone has evidence of a notable counter-claim that goes against the scientific consensus, then that is a fine basis for a question in its own right. Such a claim allows for a direct refutation and perhaps a criticism of the study on which the claim is based.
Oddthinking does make a good point in his answer, which is that consensus can be wrong. In such cases where consensus is not firmly established or still controversial it is generally easy to find evidence of a counter-claim, which brings us back to the point that counter-claims should require evidence of notability.
As I propose in my answer here, I think when notability is a point of contention that questions should be left up to the community through upvotes and comments.
If there is a question doubting a claim but no evidence of a counter-claim, than sufficient upvotes and/or comments indicating interest in or agreement with the question should be sufficient allow the question to remain.
It may also happen that the question itself may cause people who were not previously skeptical of a claim to be so, which would meet the notability requirements.
Counter-claims should not be treated different from claims and should be subject to the same notability guidelines. Having community input when it isn't clear if the claim behind a question should be considered notable is a great safeguard.
Allowing questions that are skeptical of a claim without providing evidence of a counter-claim will essentially allow questions on anything.
That is not going to be good for the growth of the site and will set a precedent for the wrong types of questions to be allowed.
Do we really want to allow questions such as asking if the moon exists regardless of if they are asked in good faith or not? Good faith is easy to feign and hard to judge accurately. It is good practice generally but on this issue it isn’t sufficient justification to allow questions without evidence of a claim.
A counter-claim is still a claim and should be subject to the same notability guidelines. If the counter-claim is not notable either by showing other people claiming it or the community indicating it is, then it should not be acceptable as the basis for a question.
this is not a place to question scientific consensus, I think this is a very tricky, and unfortunately, a politically loaded statement (though I don't think it was intended to be a politically loaded statement). I suspect we do not want to be a community for pseudoscientific attempts to debunk climate change, evolution, and the "round earth theory". But we want to allow scientific skepticism of entrenched paradigms. I'm not sure the notability standard is the right mechanism for enforcing this boundary.
Normally this isn't a problem. For most of the claims here there is going to be a 'default belief' that doesn't need evidence for its existence: for example that JFK was killed by Lee Harvey Oswald, or that wearing a tin foil hat doesn't protect you against mind control rays. But if we accept the idea that there is no need to establish notability for the opposite of a claim, it allows people to ask ridiculous questions just by reversing them.
For example we would not accept a question "Is Spain a made-up country?" because nobody believes that. However by the rules proposed in the other answers "Is the country of Spain real?" would be allowed because it's easy to find people who believe that Spain is a real place. This is obviously not a sensible approach. It opens us up to questions like: "Is the President of the US not a reptilian alien?";
