First of all, when Genesis describes the creation of "the heavens and the earth" this is language to those in the ancient Near East to mean, essentially, when God created the universe. In other words, the "heavens" were the entire universe beyond the earth, or perhaps, for some, beyond the moon, which the ancients believed was the closest celestial object to the earth. So, when God created the heavens and the earth, this included, by definition, Mercury and Pluto.
I assume that Pluto and Mercury formed at the same time, more or less as did the Earth. There are two possible views of this. One is that the timing of the events in Genesis chapter one represent literal days. If so, God does not mention every event in this account. It does not specifically mention when viruses were created, nor does it mention when Pluto or the other planets were created. Many things which were created by God are not listed in Genesis. Therefore, one is left to speculate when Pluto formed. The Bible does not say which day.
I cannot absolutely rule out the literal 24 hour interpretation of Genesis chapter one, but to be honest, I feel it is vastly more likely that the time periods described in Genesis are not literal, but represent time periods or episodes or categories of how God created the earth. This idea is sometimes called the day/age theory. In other words, the "days" represent the chronological order of creation, but not the literal amount of time. In order to explain why I feel this is a far more likely interpretation of Genesis chapter one, you will find various articles at my web site or in my book "Is There a God?" (available at www.ipibooks.com)
If one accepts that the universe is indeed quite old, then it is reasonable to assume that when gravity pulled together the matter which eventually formed our solar system, the planets, including Mercury, Venus, Earth and so forth were formed during the early history of our solar system.