Criteria for discovering Earth Like planets.

[dryson]

The real question remains on finding the first near Earth like planet is how do we set a base of programming rules for a probe to find a near Earth like planet based on the particulars of how Earth operates? Should we base the programming on how far a planet is away from a Sun compared to how far the Earth is away from the Sun? Should we look for an Earth like planet based on criteria A above where the planet would have a moon to affect the tidal forces on the bodies of water on the planet? Would the size of the sun in relation to the distance of Earth like planet when compared to the size of our Sun in relation to the distance that the Earth is from the Sun be a necessary program path to better lock down an Earth like planet?

Size of the Earth like planet in relation to the size of the sun would be important.
The distance of the Earth like planet from the sun would also be an important factor.
Another important factor would be whether or not the planet had a moon orbiting the planet or not to affect the tidal currents of the bodies of water on the planet.

Can anyone think of any other critieria?

 

[mark_d_s]

Honestly, I suspect the only criteria for potential life bearing planets would be temperature - either from the host star, from the internal structure of the planet (radioactive decay), or from tidal heating from another body (think a moon around a gas giant).

We presume that our own moon has played a role in the development of life here, but that's purely conjecture. It's difficult to draw any statistical conclusions from a sample of one.

My own personal opinion is that whilst life will probably exist in some form in another solar system, it'll be extremely rare - especially intelligent life. The reason I say this is that all the data suggests that exactly one early cell formed ~3.5 billion years ago, and we are all descended from that single entity. *IF* we find evidence that many cells developed independantly (and possibly died out) then I think that life will be very common. But that simply doesn't seem to be the case.

I think we're here today because of pure blind luck.

DISCLAIMER - I have been known to be wrong in the past!!!

 

[thnkrx]

Hmmm...sounds like you are trying to recreate the SETI target list from scratch. (Tarter and Turnbull)

From what I remember...

Star must be of the right spectral type/class.

They set an upper limit of F5 (hot) and dropped down to about M0 (small and colder). If memory serves (and it might not, the HIP catalogue gives something on the order of about 5000 stars (out of 35,000 - 50,000) within 50 parsecs that fit this criteria. Limits stars to F5-F9, G0-G9, K0-K9, M0-M1. Note that M0-M1 type stars are very faint, checking in with 1-2% of the luminosity of our Sun. Earthlike planets orbiting such stars would probably be tidally locked.
A, B, O, and T stars need not apply.

Spectral class of V or *maybe* VI (subdwarf, metallicity problems) or IV (starting to evolve off the main sequence).

Metallicity of star at least somewhat comparable to the sun (but this is a dang hard one to get right; I repeatedly ran across wildly conflicting claims (metallicity differences in excess of 100% with our sun = 100% for the same star).

Star has to be at least 3 billion years old (another tough one).

Star cannot be significantly variable (I think Tarter/Turnbull allowed for a 3% range here).

Star can be in a multiple star system...but the orbital parameters are *VERY* critical. Binary must be either very close or very widely separated. Also...most binary systems have a high degree of orbital eccentricity, which is not good for stable planetary orbits. I was a bit suspicious of the Tarter/Turnbull method for declaring a multiple star system suitable, especially those with a derth of longterm observations.

Galactic Orbit. You don't want a star with a galactic orbit that takes it ... a great deal closer... to the galactic core than we are. You get into very nasty radiation problems. Galaxy has a sort of 'habitable band' several thousand parsecs across. Now...given than galactic orbits tend to be on the order of hundreds of millions of years and there are a lot of gaps in the radiation junk in the core region (very very roughly, center third of galaxy) a earthlike planet orbiting a star with such a galactic orbit *could* get lucky a few times...but...

From there....

Claim is a 'Jupiter' (large gas giant in the outer reaches of a solar system) is either useful or essential for getting rid of the bulk of the comets and debrie that might otherwise hail down on an earthlike planet further in. If said 'Jupiter' is too big or orbits too close in, then its gravity disrupts the orbital stability of possible earths (and several of the recently discovered extra solar planets would do just exactly that.

Claim also is that a nice big moon would also be either real handy or essential to strip off 'excess atmosphere'. I'm not so sure about this.

Think you get into mass/oribtal problems if this 'earth' is actually a gas giant 'moon'.

From what I remember, HABCAT 1 gives something on the order of 17,000 possibles within a couple hundred parsecs; my own study seem to remember around 2000 within 50 parsecs.

HABCAT 2 from the TYCHO catalog gives a couple hundred thousand possibles, but this data is a *LOT* less refined; distances not known at all, spectral types, ages, ect all estimates and guesswork. (Tried using secondary sources to look into this abit, very tiring).

 

[xXTheOneRavenXx]

I'm sure it would be very hard for them to make predicting where an earth-like planet may be found possible. I mean just look at the number of stars just within 1000 ly's from earth. I am sure earth-like planets will be discovered in many places outside the predicted environments. What is to stop an earth-like (say slightly smaller than earth) from orbiting a large Jupiter type planet and life still existing on it. Who says all Jupiter-like planets give off the same amount of radiation? or that "life" on such a planet doesn't survive off the type of radiation? All stars certainly don't give off the same amount, so why should Jupter's? But I think if there is a place where life-supporting conditions do exist, a form of life will more than likely be found. It's already been proven that here on earth life can form and exist in a wide variety of conditions and environments. So why must we be so limited by the said criteria? With that said I agree that water is very important to life. But would we recognize a say desert type of planet whereas the surface is desert like, but like earth bound deserts underground streams and small pools of water provide the local creatures with what they need to survive.

 

[dryson]

Maybe a good place to start would be looking at the radiation given off by the motion of the atoms in the planets atmosphere. If we take a general account of how the Earth's atmosphere produces radiation where we then introduce the lowest wavelength of turbulence that a group of atoms are going through to the highest wavelength of turbulence that a group of atoms is going through which comprises the actual atmospheric conditions of the Earth and then overlay these atmospheric conditions to a planet that might be Earthlike. The similarities between the planet scanned might suggest that based on the atmospheric conditions of the turbulant nature of the planets atomsphere being compareable to Earth's atmosphere would suggest that the scanned planets core would be of the same active nature of the Earth's core where the magetic field that affects all atoms in and on the Earth. These events would also mean that the gravity of the scanned planet would be like that of Earth as would the amount of PSI of atomspheric pressure that would be needed on a planet for a human to be able to exist on without two much difficulty in adapting to the variances in the planets eco-sphere.

I know what I am trying to say, if anyone can help me what I am trying to say into the correct scientific terminology, the help would be appreciated.

 

[JonClarke]

What does "a base of programming rules for a probe to find a near Earth like planet" actually mean"

We don't have probes look for planets. The spacecraft we do have looking for planets beyond Earth - COROT etc. - are not autonomous.

We, that is humans on the ground, decide where they look, the search parameters used, and how the data is interpreted.

 

[robnissen]

It's already been proven that here on earth life can form and exist in a wide variety of conditions and environments.

That is NOT a true statement. It has been PROVEN that "earth life can . . . exist in a wide variety of conditions and environments." It has not been PROVEN that "earth life can FORM . . . in a wide variety of conditions and environments." It is certainly possible that earth life can only FORM in one environment and that it then migrated to exist in other environments.

 

[dryson]

What does "a base of programming rules for a probe to find a near Earth like planet" actually mean"

We don't have probes look for planets. The spacecraft we do have looking for planets beyond Earth - COROT etc. - are not autonomous.

We, that is humans on the ground, decide where they look, the search parameters used, and how the data is interpreted.

A base program would be a program that has certain criteria programmed into the CPU of the probe. When the probe scans a system or planet and encounters a result based upon the base programming the probe would continue on it's way to the next system or planet.

It might be easier explained by looking at how a robotic vacuum operates. When the vacuum is turned on and set to do it's work, the vacuum will bump into a wall or other piece of furniture, this causes the vacuum to spin in a different direction and continue it's sweeping function until it encounter's another obstacle. The same could be done with a probe. The base program would record any information from a planet as it passes by and if the information meets a certain criteria that is part of the probes database, the probe will continue on it's bouncing around the system. If the probe encounters information that is not part of the base programming the probe would gather all inforamtion available on the planet and then continue it's probing of the solar system. Once the probe has reached it's storage capacity or has expended it's fuel for a certain amount of time so that the probe could return to a transmission point where the information could then be transmitted to Earth or relay station. This type of probe would be able to be used on the outside of the solar system to gather precise information on what may be past Pluto.

 

[thnkrx]

It might be easier explained by looking at how a robotic vacuum operates. When the vacuum is turned on and set to do it's work, the vacuum will bump into a wall or other piece of furniture, this causes the vacuum to spin in a different direction and continue it's sweeping function until it encounter's another obstacle. The same could be done with a probe. The base program would record any information from a planet as it passes by and if the information meets a certain criteria that is part of the probes database, the probe will continue on it's bouncing around the system. If the probe encounters information that is not part of the base programming the probe would gather all inforamtion available on the planet and then continue it's probing of the solar system. Once the probe has reached it's storage capacity or has expended it's fuel for a certain amount of time so that the probe could return to a transmission point where the information could then be transmitted to Earth or relay station. This type of probe would be able to be used on the outside of the solar system to gather precise information on what may be past Pluto.

Nah...much simpler to identify habitable extrasolar planets ('other earths') via astronomical methods from *this* solar system. This is *almost* technically feasible *now*; it is what the whole 'terrestrial planet finder' deal is all about.

 

[SpaceTas]

Depends upon what you mean by Earth-like. With these posts there are different assumed definitions.

The simplest and broadest definition is a rocky planet about the same mass as Earth. Known examples Mercury, Venus (nearly identical to Earth in mass, radius and composition) and Mars. Observationally this would be a planet with a mass between Mars (1/2 Earth) and about 2 times Earth (fairly arbitary and excludes the already discovered Super Earths, some of which are rocky) and a radius similar to Earth. The mass and radius gives a measure of the average density and so composition.

If you start considering the possibility of life, you can tighten this definition by also requiring that the planet also orbits withing the habitable zone. This is the range of orbital distances in which the temperatures on the surface of a planet are between 0 and 100 deg C, ie where water can be liquid. This is initially calculated without considering the greenhouse effect of an atmosphere. In our solar system ranges from just inside Venus to Mars. But when you add an atmosphere this zone extends further out; imaging a bigger Mars with a thicker atmosphere ... The closest example is Gliese 581d; a super earth within this extended habitable zone. The Kepler mission will hopefully find many examples of these Earth-like planets.

Here is a link to the Gliese 581e ESO press release. It has a good image showing the habitable zone for Gliese 581 and our solar system http://www.eso.org/public/outreach/press-rel/pr-2009/pr-15-09.html

If you want a planet with life then you could consider other factors, but for microbial life it looks like all you need is a little liquid water...

If you want to detect life on a distant planet you need to do spectroscopic a polarimetric measurements, looking for signs of oxygen, chlorophyll, water bodies, ...

 

[MeteorWayne]

Relevant SDC article:

"When scientists search the heavens for habitable worlds beyond Earth, they don't necessarily know what to look for. A new study has found that the most probable place to find intelligent life in the galaxy is around stars with roughly the mass of the sun, and surface temperatures between 5,300 and 6,000 Kelvin (9,100 and 10,300 degrees Fahrenheit) - in fact, stars very similar to our own sun.

Learning that sun-like stars are good candidates for life may not sound surprising, but it isn't always what scientists have thought."
...........

The researchers weighed these factors against each other to calculate the distribution of stars most likely to host thinking, living creatures. "It's a tradeoff between the numbers of stars out there and the probability of habitable planet formation increasing with mass." Whitmire said. "We show it's no accident we find ourselves around a star like the sun." The distinction between habitable planets and planets harboring intelligent life is based on the fact that intelligent life requires stars with lifetimes greater than the time required for intelligence to evolve. For example, in the case of this solar system, we could not find ourselves around a star with a lifetime less than 4.5 billion years.

Indeed, sun-like stars seem to have the right balance: They are of high enough mass that they are more likely to host habitable planets, but they are of low enough mass that they live long enough for intelligent life to develop, and are not extremely scarce. Whitmire estimates that 10 percent of the Milky Way's stars might fall into the category they've outlined. This would still leave over 10 billion candidate stars in the Milky Way alone."

The study is detailed in the September 2009 issue of the Astrobiology Journal.


more here....

http://www.space.com/scienceastronomy/0 ... liens.html

 

[xXTheOneRavenXx]

It's already been proven that here on earth life can form and exist in a wide variety of conditions and environments.

That is NOT a true statement. It has been PROVEN that "earth life can . . . exist in a wide variety of conditions and environments." It has not been PROVEN that "earth life can FORM . . . in a wide variety of conditions and environments." It is certainly possible that earth life can only FORM in one environment and that it then migrated to exist in other environments.

Thank you for that correction robnissen. I should have said "exist" instead of "form" as you said.