Prevalence of Earth-size planets orbiting Sun-like stars

[Chronos]

This paper, http://arxiv.org/abs/1311.6806, discusses Kepler mission results. The authors deduce that 5.7% of sun-like stars have an Earth sized planet orbiting in the habitable zone. That is a shockingly large number, IMO. Considering there are an estimated 26384 class G class stars within 324 light years of Earth [re: http://www.astronomycafe.net/qadir/BackTo110.html], that means there are about 1500 Earth sized planets that orbit in the habitable zone of class G stars within 324 light years. The study considers both class G and K stars as sun-like, so the number of such planets should more than double, since class K stars are even more come common than class G stars.

 

[Damo ET]

This paper, http://arxiv.org/abs/1311.6806, discusses Kepler mission results. The authors deduce that 5.7% of sun-like stars have an Earth sized planet orbiting in the habitable zone. That is a shockingly large number, IMO.

Chronos, why would you consider 5.7% surprising? What would you have thought would have been a more representative percentage?



Damo

 

[Chronos]

All things considered, I suppose it isn't. I recall not so many years ago that was about the estimated fraction of stars that had any planets. That number was obviously very pessimistic. My offhand guess would have been about 10% of all stars had an Earth sized planet, and a significantly lower fraction of those in the habitable zone. In fact, I thought that was fairly generous. Again, obviously not. While perusing the literature I ran across another, even more shocking number - Earth sized planets in the habitable zone of class M stars is estimated to be 51% to 61%: A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs, http://arxiv.org/abs/1303.2649

 

[Bobbywhy]

Chronos, just one "off the wall" question, please: With all these "terrestrial planets in habitable zones" can you speculate about the possibility of some extraterrestrial life on any of those? I knew some of the original SETI team folks and remember their thoughts on this. Thank you.
Bobbywhy

 

[Damo ET]

Although those numbers are quite large, they don't seem to be too optimistic. I would think that most 'M' stars with a similar metalicity to the sun would have had a similar materials in the accretion disc, which would have lasted longer before the star ignited. The additional time should give the rocky planets better chance of forming in a higher number closer into the star. Although the habitable zone will be smaller with the type 'M' compared to 'ours', I would expect the number of planets in the zone would still work out to be greater than for a star like the sun. Is that line of thought feasible?

As for life, I would think the biggest 'low probability' characteristic of the Earth compared to other Earth sized worlds, would be the size of the Earth's iron core. If I have understood correctly, in the early solar system formation when the young molten Earth was struck with another molten proto planet, both the cores merged and most of the ejecta collected to form the moon. This gave 'us' an unusually large iron core which still gives 'us' the relatively strong magnetic field today that protects against the solar wind. Without the extra iron from that impact, we would have probably ended up like Mars without a substantial atmosphere that can be held onto due to loosing its magnetosphere.
Then there is the role that the moon played with the tides and whether it was a factor in giving life a boost.



Damo

 

[jim mcnamara]

I do not buy your 'iron core' argument because one of the explanations for Mars' loss of a magnetosphere 4 billion years ago [ http://science1.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ ] is a huge late meteor impact [ http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html and http://www.theguardian.com/science/2008/jun/26/mars.asteroid?gusrc=rss&feed=science ] that apparently disrupted the core.

What this means to me is Mars once was one a more Earthlike planet. It got clobbered and was transformed into what it is today. Do you have any citations to back up your point of view?

I'm not saying there is a one perfect hypothesis. Also note that the distribution of elements in Mars geology is different from Earth - due to Mars physical location in the Solar system. Lighter elements like chlorine are more prevalent than here on Earth. This is the result of Mars accretion disk losing less lighter material because solar wind intensity is reduced.

 

[Damo ET]

I do not buy your 'iron core' argument because one of the explanations for Mars' loss of a magnetosphere 4 billion years ago [ http://science1.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ ] is a huge late meteor impact [ http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html and http://www.theguardian.com/science/2008/jun/26/mars.asteroid?gusrc=rss&feed=science ] that apparently disrupted the core.

What this means to me is Mars once was one a more Earthlike planet. It got clobbered and was transformed into what it is today. Do you have any citations to back up your point of view?

I'm not saying there is a one perfect hypothesis. Also note that the distribution of elements in Mars geology is different from Earth - due to Mars physical location in the Solar system. Lighter elements like chlorine are more prevalent than here on Earth. This is the result of Mars accretion disk losing less lighter material because solar wind intensity is reduced.

I have extrapolated some of the information myself from different sources I have come across in the past which was why I stated 'If I understand correctly'. Brian Cox has spoken about the lack of a substantial Martian magnetic field due to the solidification of the core, Dr Pamela Gay has spoken about both the Earth an Mars in her Podcasts on Astronomycast ref#17 #51 and #52. All the general terrestrial planet Wiki articles indicate that the Earth has a substantially larger core than would have been expected for a body of its size forming at the same time as the other terrestrial planets. 3 out of the 4 large bodies in our habitable zone have solid iron cores preventing a substantial enough magnetosphere to retain a nitrogen/oxygen atmosphere when exposed to the solar wind, Earth being the exception. The difference being that the Earths is the biggest.

BTW, the first article you reference indicates that Mars had already lost its magnet field before the big impacts.

Damo

 

[Chronos]

It is my understanding that Mars still had a reasonably thick atmosphere 1 billion years ago, and probably liquid water. It was a harsh environment, but, some extremophiles may have existed. Finding evidence thereof remains elusive. Based on Earth's biological history it is probably unlikely anything much more complex than bacteria could have evolved.

 

[Chronos]

Regarding the prospects for life on Earth sized planets in the habitable zone, liquid water is the key. IMO, the prospects for life on any habitable planet with an abundant water supply are very favorable. The prospects for complex life are completely unknown, but, I believe many such planets would harbor simple life forms.

 

[Damo ET]

I agree 100% Chronos, if there is liquid water and there is a timescale of at least 1 billion years, I would expect life to be the norm rather than the exception. As you suggest, you can literally look where you like on our planet whether it be smokers on the sea floor, the driest of the dry Antarctic desert or man made toxic waste lakes, life seems to be relentless and unstopable here. There seem to be quite a few better candidates elsewhere in our solar system when you compare to where life has been found here.

I wasn't aware that Mars still had a substantial atmosphere as little as 1 billion year ago! I would be shocked if the current Mars rover 'Opportunity' doesn't show up bacterial remains in samples. Damo