I was looking at the Collins Cobuild Compact English Learner's Dictionary for its explanation and example sentences accompanying its explanation, of the term N-PROPER, which I presume is referring to a proper noun. An example on the proper noun 'Senate' went like this:

Senate: All of the president's cabinet appointments must be confirmed by the Senate.

At the same said dictionary, I found the following definition for the proper noun word 'heaven':

heaven: N-PROPER In some religions, heaven is said to be the place where where God lives and where good people go when they die.

At the same said dictionary, One definition for the proper noun word 'Earth' is this:

earth: N-PROPER The Earth is the planet on which we live.

What I am trying to get at is this: Why some proper nouns always seem to accompany the definite article 'the', whereas some proper nouns don't? Usually, you wouldn't see the word 'Senate' without 'the', nor would you see the capitalized word 'earth' without 'the' if one wants use it to mean the planet we live in. Wouldn't it?
Some proper names have "the" as part of the name, and some don't.

The United States of America (or the U .S.) has a "the." Mexico and Canada do not.

As a noun, "the Senate" will have "the." (As an adjective - "Senate-appointed prosecuter" - it will not.)
Hi Believer

In addition to what GG has said: people often use the article with earth and don't even capitalize the word, as in:

A thin atmosphere encircles the earth.

However, in scienctific English all our planets are capitalized and used without the:

Earth is bigger than Mercury.
Jupiter is the biggest of the planets.