What does it take for a planet to be "earthlike"

[heyscottie]

This thread is inspired by some of the discussions in the Gliese 581C thread. I'll just ask the question without offering any opinions first.<br /><br />What characteristics must be present for us to term a planet "earthlike"? Surely there are different levels of similarity. Perhaps we would like to set up an ordering of most important characteristics first. For instance, a less important characteristic would probably be something like "contains a large moon to raise tides", where a more significant characteristic would be something like "supports liquid water".<br /><br />So have at it!

 

[ctrlaltdel]

When I think of an earthlike planet, I first thought of somewhere humans and other high order lifeforms can live without a pressure suit/breathing gear. But that goes beyond the need for a certain atmospheric composition and pressure. Would the ecosystem of this planet contain all the nutrients essential for life? Could many of the organisms contain harmful compounds (there are organisms on earth that rely on the reduction of arsenic compounds, for example)?<br />

 

[origin]

About the only requirements I would think would be that water can exist on the planets surface in liquid form. <div class="Discussion_UserSignature"> </div>

 

[ctrlaltdel]

"About the only requirements I would think would be that water can exist on the planets surface in liquid form."<br /><br />That can be achieved at very high temperatures if the atmospheric pressure is high enough.<br />Our hypothetical planet would also have to undergo a similar event to our own oxygen catstrophe before we could breathe the air. If the planets ecosystem fails to progress beyond such a primordial state, then does it still fit the description of earthlike?

 

[docm]

A nitrogen, argon or other light inert (or mostly so) gas based atmosphere w/o toxic impurities, O2 between 10% and 30%, pressure between .3 and 1.5 atmospheres (inverse relative to the O2 pct.), liquid water @ summer temps, .5 to 2.0 G's and mid latitude temps between 0F and 120F. <div class="Discussion_UserSignature"> </div>

 

[origin]

<font color="yellow">"About the only requirements I would think would be that water can exist on the planets surface in liquid form." <br /><br />That can be achieved at very high temperatures if the atmospheric pressure is high enough. <br />Our hypothetical planet would also have to undergo a similar event to our own oxygen catstrophe before we could breathe the air. If the planets ecosystem fails to progress beyond such a primordial state, then does it still fit the description of earthlike?</font><br /><br />I would say yes. I think that earth like means a rocky planet that is capable of supporting life. If there is water in liquid form on the surface, then these requirements are met. Just my opinion. <div class="Discussion_UserSignature"> </div>

 

[aaron38]

I have a rather basic, utilitarian definition of "earthlike".<br />The planet would have to be capable of sustaining a human civilization who possess only a minimal level of technology.<br />No oxygen generators, no airlocks, no water reclamation systems. No electricity.<br />Also no widespread toxins in the environment.<br /><br />Life may be incredibly harsh. Only portions of the surface may be habitable. Water may be hard to get to. But a human would have to be able to live unprotected on the surface.<br />If the planet is truly earthlike there will be edible plantlife. Although as long as the soil can support transplanted plantlife with greenhouses then I think that counts.

 

[vandivx]

it all depends on your time horizon, at one point Earth was glowing magma with no atmoshpere and no liquids whatsoever, just potential for them - and what is more Earth-like than this LOL<br /><br />there is no fixed definition and to settle on something we woud have to be given some constraints<br /><br />some planet found around other star might be called Earth-like if it orbited the star in certain fashion but that doesn't mean we would see it as Earth-like for the purpose of making trip there with the goal of landing and perhaps even staying<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

 

[qso1]

Currently, earthlike is generally used to describe a rocky world as opposed to gas giant. At least when talking exoplanets. In this solar system, Venus has been called earths twin, although that was heard more before space probes revealed what Venus was really like. Nowadays, if the twin analogy is invoked at all, it describes as twin because its a rock nearly the size of earth with a significant atmosphere. Mars is called the closest planet we have to being an earthlike world in our system.<br /><br />Little can be determined about the true nature of the rocky worlds discovered around Gliese581. Does 581c have water? Maybe not despite being in the goldilocks zone of orbital distance from G581.<br /><br />Once fairly detailed probably second or third generation images of these worlds come in, we will very likely find one or more that are strikingly earthlike. This would cause a redefinition of what constitutes an earthlike world much like the recent IAU debate over what constitutes planets.<br /><br />I suspect earthlike will eventually be narrowed to planets with environments resembling earths more closely than mars and venus do. There would then be venuslike and mars like worlds to deal with as well.<br /><br />Imagine a planet more earthlike than mars but still largely desert and having maybe an atmosphere around 5 percent of earths atmospheric pressure at the surface. Some O2 present but mostly CO2 and probably not breathable for humans.<br /><br />Naturally this world would replace mars as the most earthlike world to date while still being a little more marslike than earth from a human visitation perspective.<br /><br />Then one day we find a planet clearly eartlike in almost every way, right down to being able to see evidence of cloud cover, oceans, continents, and most importantly...a spectrum that nearly matches earth.<br /><br />A planet that may one day become the first world in which humans may breath alien air. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[qso1]

A sample of potential future earthlike worlds that could help narrow the definition of earthlike. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[robnissen]

Here is an article in slate on what it means to be earth-like (previously posted in another thread):<br />http://www.slate.com/id/2165044/

 

[qso1]

As usual, beaten to the punch again.<br /><br />I've been working on an article myself that was to begin to address this issue but as always...beaten. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[heyscottie]

Here are a few characteristics, in what I consider order of importance, for long term human habitation.<br /><br />Most Important<br />----------------------------------<br />1) Rocky planet or moon<br />2) Surface gravity "reasonably close" to g<br />3) Lies within habitable zone where water stays liquid<br />4) Maintains magnetic field to repel radiation<br />5) Host star not variable<br />6) Planet contains large amounts of water<br />7) Has atmosphere close to 1 atm of pressure<br />8) Atmosphere contains free oxygen (this almost certainly implies life already exists there)<br />9) Atmosphere largely free of toxic gasses<br />10) Planet contains dry land<br />11) Abundant in minerals and building materials<br />12) Experiences day/night cycle close to 24 hours in length<br />13) Axial tilt not too great so seasons not too extreme<br />14) Orbit not too eccentric so seasons not too extreme<br />15) Host star a yellow, sun-type star<br />----------------------------------<br />Least Important<br />

 

[vandivx]

some of those requirements seem to beg the question - does earthlike mean generally suitable for life development or suitable for human like life development or potential future settlement, you seem to go by the latter premise<br /><br />for example I can quite imagine large departure from the g requirement, I believe life can adapt to more severe conditions than we think<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

 

[qso1]

Excellent list. Of course there are a few on the list I would address in the "We might be surprised" category.<br /><br />Dry land desirable but maybe not required. A planet covered in water could harbor an almost infinite amount of sea type life and one day this planet may not always be covered in water in which dry land would become present along with creatures that migrate from the oceans to that land.<br /><br />Day/night cycle might not be an absolute considering life right here on earth at near polar latitudes does not seem to mind living under the midnight sun as it were.<br /><br />Axial tilt we may find is not perhaps as conducive to life as our axial tilt but I tend to think axial tilt will be more of a factor in where life develops in abundance. An example would be say, an earthlike world tilted nearly 90 degrees, the north polar regions pointing towards the host star. This puts the south polar region in perpetual darkness (In a single star system). Life would probably best survive in a band around the equator or more accurately, the terminator shaded portion of the planet. The night side being too cold, the day polar regions being too hot. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[heyscottie]

Yes...I was building my list based on human habitation, not on formation of life.<br /><br />Formation of life, and also formation of intelligent life, would require a far less restrictive list than the one I wrote for humans. Anyone want to take a stab at one?