Personally, I think trying to pigeon-hole planets discovered by Kepler as “Earth-like” versus “Venus-like” is an exercise in nit-picking.

I disagree because it makes the, I think, rather important different between a habitable (well for Humans) planet and an uninhabitable one. For me the word “earth like” conjures up a planet that is, well, like our Earth. In more ways than just mass and radius.

Venus has an almost identical size and mass as Earth but few would call conditions there “earthlike” at all. Venus is, in fact, perhaps the most hellishly hostile and unearthly planet in our solar system – certainly out of the four rock dwarf “terrestrial” planets.

Considering that Kepler isn’t actually able to distinguish the difference between planets like Earth and planets like Venus, and if there was an alien Kepler mission looking at Sol, if it could detect Earth and Venus at all, those alien astronomers would not be able to predict with any great certainty which one of the two is like Earth and which one is like Venus, and would most assuredly classify both planets into the same category,

Or would they? Interesting point there but I’m not so sure its really accurate. Such aliens may not be able to tell with complete certainty – assuming a technology level the same as ours – but they and we can certainly calculate and make educated guesses based on the planets size, distance, sun type, the likely effects of probable atmospheres , etc..

I’m guessing if we found a system like our own with two earth-mass exoplanets equivalent to where Venus and Earth are from a star the same spectral type, luminosity and age as our sun we could correctly deduce that the inner one was Venus-like and the outer one was earth-like. If we could do that it makes sense to me that equally intelligent aliens could do so as well.

I think that for the purpose of considering Kepler planets, Venus-like worlds ARE Earth-like worlds.

That’s rather a subjective assessment. I think calling a world that is like Venus otr Mercury or Mustafar “earth-like” creates the wrong impressions in the general public and is very misleading and I wish we would describe things just that bit more accurately so that a world that is described as “earthlike” is a very probably habitable planet where we could live comfortably without spacesuits in earth-like temperatres and pressures. Conversely, a world that is more like Venus or a supersized version of Mercury or Mustafar in surface conditions is , I think, better described as Venus-like / Mercury-like / Mustafar -like.

We often call Venus Earth’s “evil twin”, and these days people tend to focus on the evil part, but we seem to forget the second word there all too often.

Perhaps so, yeah. The planet named for the goddess of love seems to get precious little from us sometimes! But that doesn’t change what I’ve said above. I see what your getting at I think but I don’t really agree.

I don’t think that follows. If the earth had no liquid oceans or atmosphere ,and a day lasted for a couple of years, then I could see taking the temperature of the night side, comparing it to the day side, and calling that the sun’s contribution to average surface temperatures. But night time isn’t all that long. If it were, I’m sure you’d see much, much greater temperature changes between night and day. I’m sure decay heat is a measurable component, but I can’t imagine it’s that much.
Look at Jupiter’s moon Io. It experiences a whole lot of tidal heating, driving its famously prolific volcanism, and its mean surface temperature is about 110k.

Same point but different presentation: The Earth temp varies. It is higher by up to 10-15C in daytime than at night. Hence solar insolation probably accounts for a max of 15C or 5% out of the total 289K. Decay heat apparently is 52% of Earths heat (Nature Geoscience July 2011) and if Solar E/M radiation is a max of 5%, where does the rest come from? Could it be that the force of eccentric gravity from Jupiter sized (& all other)planets actually adds and subtracts energy (force of gravity causes angular momentum, or spin and if the planet has layers (solid core, liquid core, and atmospheres) this spin causes friction which tranports gravity into temperature Heat So if these are three different sources of energy on a planet does the item 22 mechanism for caluculating the temperature still work?
How about comparing Venus & Earth? Venus has a surface pressure 90+ that of earth, Does this impact the temp? PV=nRT says yes. So Does the #22 mechanism still work?.
Regardless of these trivial little validity questions, Congrats to Kepler team & ALSO to #9 & 22 for thinking of the questions & answers. Keep thinking!

Thanks for your response. I don’t doubt your numbers as I’m familiar enough with the relationship between telescope size to know even the Keck telescopes would be unable to meet the 100×100 challenge. A 100×100 image would be quite detailed. I think a 25×25 would be a great starter.

After searching for space based interferometry I came across this http://en.wikipedia.org/wiki/Astronomical_interferometer#Labeyrie.27s_hypertelescope

I should have done more reading before posting my question. Here are some possible missions to image other planets.

http://en.wikipedia.org/wiki/Darwin_%28ESA%29
http://en.wikipedia.org/wiki/Terrestrial_Planet_Finder

Also given their locations brushing up against their suns ferocious surfaces calling them Mustafar class worlds or SuperVenuses or SuperMercuries is certainly more likely to match their nature and character.

Personally, I think trying to pigeon-hole planets discovered by Kepler as “Earth-like” versus “Venus-like” is an exercise in nit-picking.

Considering that Kepler isn’t actually able to distinguish the difference between planets like Earth and planets like Venus, and if there was an alien Kepler mission looking at Sol, if it could detect Earth and Venus at all, those alien astronomers would not be able to predict with any great certainty which one of the two is like Earth and which one is like Venus, and would most assuredly classify both planets into the same category, I think that for the purpose of considering Kepler planets, Venus-like worlds ARE Earth-like worlds.

We often call Venus Earth’s “evil twin”, and these days people tend to focus on the evil part, but we seem to forget the second word there all too often.

It is highly possible that Kepler 20 has other planets further out then these. These will be harder and take longer to find as their “years” will be commensurately longer.

I wonder if this might end up being the solution to the apparent paradox of the planets’ size/distance from star distribution. Given that what we’ve got here are two earth-sized, one super-earth/mini-Neptune, and two Neptune/Uranus sized worlds here. The mystery of the big-small-big-small-big distribution might end up simply actually being a mirage, with the system actually being small-small-small-small-small, with bigger Jupiter class worlds further out yet to be discovered.

After all, I believe we know of several other multiplanet systems where there is at least one gas giant closer in than smaller, super-earth class planets.

Why does it matter if the planet is the same size as earth?

It demonstrates unequivocably that Kepler really can (is sensitive enough) to detect planets that small, as we had hoped, but until now had not known for certain.