No, there is not meaning of them in TV series till now.
It is noted as Trivia in Wikia Howard wears alien pins on his collar in every episode, yet the pin has not been mentioned thus far.
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To expand panthers answer, actually they confirmed that the pin has a meaning, but it is a secret.
It is not only one pin, but different pins with different shape and color, but all showing an alien.
You can find several discussion and theories of fans about the theme in the net, but no confirmed answer.
There is a short behind the scenes video about this with the actor here:
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Possible Reason #1 Being regular viewer of TBBT I could say this that we are missing a big fact here.
Howard is an astronaut I think the pin depicting the alien is a badge that he wears for being out of space or at least traveled in space.
As in our modern society someone coming from space is usually considered to be an alien .
Possible Reason #2 Apart from Howard using that as pin that symbol of alien is also logo of Alienware which is also the brand Sheldon uses.
and Howard also owns one computer of same brand. .
So maybe it's about branding.
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Howard says in season 3 when they go through the box where they find the ring (this is a paraphrase
I don't remember the exact quote)
what he says is I use to have one of these dolls I pretended my dad was ubtucted by aliens and that this guy would bring him back.
So Howard wears it because of his childhood life so he would remember to take a step back and be a kid again
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The show is about disclosure and acceptance.
The show might be a well thought out way to show aliens for the first time.
There are many references used subtly throughout the series.
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Besides skinny jeans and a wide variety of belt buckles, one of Howard's signature items in  The Big Bang Theory  is an alien lapel pin. Is there any Reason or Meaning behind it?
What's the meaning behind Howard's alien pin?
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The show might be a well thought out way to show aliens for the first time There are many subtle references throughout the series. The show might be a well thought out way to show aliens for the first time.
The show is about disclosure and acceptance. The show might be a well thought out way to show aliens for the first time.