how likely it is that it will happen

[Maybo]

What’s the difference between the following?
1. Besides each worry, explore how likely it is that it will happen
2. Besides each worry, explore how likely it will happen

 

[Barque]

What’s the difference between the following?
1. Besides each worry, explore how likely it is that it will happen
2. Besides each worry, explore how likely it will happen

The second is wrong.

Also, the original uses "beside" (to mean "by the side of") while you've changed it to "besides" (which means "apart from").

 

[Maybo]

I made mistake. It should be:
1. Beside each worry, explore how likely it is that it will happen
2. Beside each worry, explore how likely it will happen

Why do we need "it is that"?

 

[Barque]

"Likely" is an adjective. It qualifies the dummy "it" in the first option.

In the second, you're using "likely" as an adverb. While it can be used as an adverb, that isn't the correct structure for it to be used as an adverb. It's not said that way. Sometimes the reason a structure is wrong is that it's just not used, as in this case.

It will happen.
How likely is it?
That amounts to How likely is it that it will happen?

 

[emsr2d2]

You could use:

... explore how likely it is.
... explore how likely it is to happen.