Let's first define "objective".
"Objective" means that a statement is not dependent on the subject that made it to be true.
(of a person or their judgement) not influenced by personal feelings or opinions in considering and representing facts."
https://www.lexico.com/en/definition/objective (Oxford Dictionary)
Let's define "opinion".
A jugemental view someone holds that has been concluded from subjective information (like feelings or thoughts) or objective information (like measurements or observations).
a view or judgement formed about something, not necessarily based on fact or knowledge."
https://www.lexico.com/en/definition/opinion (Oxford Dictionary)
"This icecream is tasty, in my opinion." is a jugemental statement based on the things you felt. Thus it is inherently subjective. Making it by definition a subjective opinion. It may not be proven wrong due to its subjectivity.
"Pluto is a planet, in my opinion." is a jugemental statement based on measureable information. From the measurements, there are words to define what qualifies as a "planet". This is called the "objective criteria" and it may be satisfied to reach the conclusion. The fact that the statement is seperate from the subject makes it an objective opinion.
Of course the question if it is true can be discussed (the truth of the statement is of no concern in this instance). Therefore it is not an universally agreed fact but instead a judgemental view - an opinion.
To say "in my opinion" is redundant here and not often used when talking about objective opinions. That does not not make them opinions.
To clarify the confusion about objectivity and fact, I will use more abstract terms in an attempt to prove that the difference exists.
Let A, B, C be indisputable objective facts (they are here defined to be true).
Let D be a jugemental statement.
Let "A and B therefore D" be a logical statement.
Let "A and B and C therefore not D" be a logical statement.
Therefore if C is true, then not D.
Let P be a person.
P is only aware of A and B therefore P concludes D is true.
A and B are objective therefore D is objective.
Since C is true, D is wrong and therefore not a fact even though P concluded objectively that D is true.