[MeganM (OP)]

Viv has asked:

Do scientists think the Earth's orbital distance is optimal for life, or could we do with being a bit nearer or further away from the sun?

Thoughts?

[Halc]

It's not that Earths orbital distance is optimal for life, but rather that life is optimal for the Earth's orbital distance. Were the Earth to be at some other radius, different life would have evolved that preferred the new conditions.

That said, the above statement assumes there is already life, and the question might instead be interpreted as the radius optimal for abiogenesis, assuming abiogenesis actually occurred on Earth.  In that case, we'd probably need to concede that it's nice that there is liquid water here. The existence of that probably helped life to form in the first place, so an radius where the water is all ice or all boils away is probably less optimal for life, and not just life as we know it.

[alancalverd]

In a word, obviously. But as Halc implies, the egg preceded the chicken. Life evolved at this radius and a particular phase in the cooling of the planet. My own feeling is that Mars was once optimal but life couldn't evolve fast enough to cope with the loss of atmosphere. The same will eventually apply on Earth but on a much longer timescale. 

[Outcast]

The slightly elliptical orbit of the Earth does produce "a bit" of variation in the distance...5000 km or 3.3 percent. This is within the "Goldilocks Zone"...not too hot, not too cold. It's a matter of some debate whether an orbit in the Goldilocks Zone is "necessary, but not sufficient" to support the genesis and evolution of life.
There are 181 moons in the solar system. Ours is BY FAR the largest relative to its' host planet. No moon means no tidal pools, the classical theory of the location of the genesis of life. Would a smaller moon serve as well? Nor are tidal influences the only consideration.
The moon's size is also credited with stabilizing the rotational axis of the Earth (though not entirely...the axis still precesses in a 28,000 year cycle). Absent a stabilizing moon, there would be a significantly higher rate of precession, with a corresponding disruption of the seasons on Earth. The effects this would have on the beginnings of life and evolution are largely unknown.
What's clear, (at least to me,) is that life, and certainly us, are not inevitable in the Goldilocks Zone.

[Halc]

There are 181 moons in the solar system. Ours is BY FAR the largest relative to its' host planet.

Our moon is about 1% the mass of its primary.  Charon is about 12% the mass of its primary.
Your statement is arguably true since Charon's primary was demoted from planet status, but that 'by far' in caps at least deserves an asterisk then.

[Petrochemicals]

Discounting radiation etc, I think that the thing that enables life is the quick rotational speed of earth coupled with the strange properties of water as its refrigerant. The axial tilt adds to this temperature evening.  Water would come to a similar temperature level relating to the coldness of space over a large orbital distance variance. It doesnt need a planet of a set size, other than with the gravitational ability to hold on to its atmosphere. Liquid water is (as you probably know) what is believed to have given rise to life. Water has very high heat capacity, evapouration and liquidity latent energies, so its range as a refrigerant with space as a cooling medium is fairly narrow, energy holding is large and increaced energy input is cushioned.

Due to the planet rotational motion, the water is kept liquid over a large area and the temperature is kept within a small margain, not boiling one side of the planet and frozen the other. This allows the water to act as a refrigerant bringing the temperature to a level that is moderate and general planetwide.

[Outcast]

Some research has suggested that beyond a Goldilocks zone in our solar system, there is also a galactic Goldilocks zone, not based on "not too hot, not too cold,"  but "not too dense, not too sparse" star population.  A star too near the center (dense population of stars) of the galaxy is subject to all manner of intense radiation inhospitable to our form of life on any planets that form around it, as well as extreme gravitational forces.
A star too far from the center (sparse population of stars) of the galaxy will not have the history of supernovae incidence we believe necessary for the creation and distribution of heavier elements.