The data from Kepler’s first 136 days of operation could not be more interesting. As discussed in yesterday’s news conference, we now have fully 1235 exoplanet candidates from Kepler’s transit observations, and it’s worth quoting principal investigator William Borucki (NASA Ames) on the significance of the results thus far:
“We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone – a region where liquid water could exist on a planet’s surface. Some candidates could even have moons with liquid water. Five of the planetary candidates are both near Earth-size and orbit in the habitable zone of their parent stars.”
Statistical analysis by the Kepler team shows that between 80 and 90 percent of these candidates are likely to be real planets. Remember that the spacecraft is staring at 156,453 stars in a patch covering 1/400th of the sky, located in the constellations Cygnus and Lyra. What it’s giving us is a statistical sample of stars in a particular part of the Milky Way, one we can use to extrapolate planetary populations throughout the galaxy. In a helpful post, Franck Marchis (UC-Berkeley) lays out the properties of these planets classified by size:
- 68 Earth-size exoplanets with a radius (Rp) of less than 1.25 Earth radius (Re)
- 288 super-Earth size exoplanets with 1.25 x Re < Rp ? 2.0 x Re
- 662 Neptune-size exoplanets with 2.0 x Re < Rp ? 6.0 x Re
- 165 Jupiter-size exoplanets with 6.0 x Re < Rp ? 15 x Re
- 19 very-large-size with 15.0 x Re < Rp ? 22 x Re
The numbers are useful as well as deeply exciting, because they suggest how vast the exoplanet population must be on the galactic scale. Kepler is seeing only the small fraction of planets whose orbital alignment as seen from Earth causes them to transit their hosts stars. Roger Hunter, Kepler project manager, sums up what many had long suspected but lacked evidence for until now: “It’s looking like the galaxy may be littered with many planets.”
Image: Kepler’s planet candidates by size. Credit: NASA/Wendy Stenzel.
Where is Earth 2?
One of the questioners at yesterday’s news conference asked Douglas Hudgins, a Kepler program scientist, whether buried within the latest data release the project’s ‘holy grail’ might be lurking; i.e., a planet in the habitable zone of a star like the Sun. We do see planets in the habitable zone of certain stars from Kepler’s work, but Hudgins had to point out that the true ‘grail’ Kepler was after was a planet similar enough to Earth that it would be found in a roughly one-year orbit around a star like ours, and squarely in the habitable zone. We haven’t yet had enough time for such an observation to be made, since it would take more than one transit to reveal such a recurring event.
So right now we’re talking about planets in the habitable zone of stars that are cooler and smaller than the Sun, places where the star in the sky would be a lot redder than the Sun as seen from Earth. The next data releases will get more and more intriguing on that score as we home in on true Earth-analogs. At that point Kepler’s statistics really will be giving us a glimpse of the likelihood of the kind of planets we could live on and how common they are in the galaxy. As the overall catalog grows, it will become more accurate and should reveal not just exoplanets orbiting at larger distances from their host stars but very possibly moons around some of them.
We’re also getting a better read on just how skewed our early exoplanet results were toward large planets, particularly the so-called ‘hot Jupiters.’ That was an inevitable result of picking off planets that were the easiest to detect, but Kepler is now showing us that stars in our galaxy are more likely to be orbited by smaller worlds. Let me quote Marchis on this:
…we can now say that stars in our Milky Way galaxy are more likely to host small exoplanets since 75% of the Kepler catalog exoplanets are smaller than Neptune, with a peak of exoplanets only 2-3 times larger than Earth.
Using model predictions which take into account the probability of having the correct geometry to detect these exoplanets, the Kepler team extrapolated that 6% of the stars in our Milky way have Earth- and super-Earth size exoplanets, 17% of them have Neptune-size candidates and only 4% of them have Jupiter-size exoplanets.
Image: Kepler’s planet candidates as of Feb. 1, 2011. Credit: NASA/Wendy Stenzel.
Why so comparatively few multi-planet systems (about 170 thus far)? Considering the short period of time Kepler has been in operation, we’re working with fairly short orbital periods. That’s one thing that makes Kepler-11, discussed here yesterday, so unusual. Given that we’re talking about orbits close to the parent star, it’s astounding to find a star with five planets crammed inside the orbital distance of Mercury, and a sixth at a distance between Mercury and Venus around our Sun. The five inner planets have orbital periods varying between a scant 10 to 47 days around this G-class star, and from all the indications we have, the system is dynamically stable.
Bringing the Hunt Back to Earth
Where would we have gone next in our exoplanet hunt if money were no object? The Space Interferometry Mission immediately comes to mind, because while Kepler could give us a statistical look at a particular patch of sky, SIM would have targeted nearby stars, giving us detections via interferometry of Earth-like worlds within thirty light years. Without SIM or, for that matter, Terrestrial Planet Finder, we’re finding other ways of locating such worlds from Earth’s surface. Ground-based surveys like MEarth and new technologies like laser frequency combs may help us fill out our target lists for the upcoming James Webb Space Telescope.
Lee Billings looks at these technologies in a fine new article in Nature. Take Steven Vogt and colleagues, who have built the Automated Planet Finder (APF), a robotic telescope paired with a powerful spectrometer at Lick Observatory in California. Like MEarth, APF targets short period planets around nearby stars, and that includes the brightest M-dwarfs in the sky. One way to save money while still hunting exoplanets is to maximize observing time. Billings quotes Vogt:
“The coin of the realm is observing nights,” he says. “It’s not new technology; it’s not laser combs or some newfangled near-infrared spectrometers that can take advantage of M-dwarfs. Take $50 million, which is chump change in the NASA regime, build a 6–8-metre telescope with enough light-gathering power to reach a large fraction of the nearest M-dwarfs, put a nice spectrometer on it and dedicate it to this work every single night of the year. You’d have these planets pouring out of the sky.”
And maybe there’s a cheaper way in space, too, as revealed in Sara Seager’s ExoplanetSat program. Seager (MIT) has the notion of entire fleets of tiny CubeSats, dozens at a time scanning individual stars, each satellite with its own particular target. Planets around nearby Sun-like stars should be detectable if they transit, and Billings says we should see a functional prototype as early as 2012, with subsequent satellites launching for as little as $250K apiece. So there are ways to proceed beyond Kepler, and they’ll surely be in full gear even as the Kepler data continue to arrive and the planets we discover begin more and more to resemble our own.
For more, see Ford et al., “Transit Timing Observations from Kepler: I. Statistical Analysis of the First Four Months” (preprint) and Borucki et al., “Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data” (preprint).
I know I’ve asked this before. But if you can point me to a good paper I can download, I won’t ask it again.
Most of these solar systems seem to have gas or ice planets that migrate inward. Our solar system, in contrast, seems to have kept its giant planets outside the snow line and has allowed the rocky planets (including us) in stable orbits for several billion years.
Why is this migration phenomenon not seen in our solar system when it seems to be near universal for most of the systems we’ve found so far?
Hardly anybody here is mentioning the issue of metallicity, always considered to be so relevant in relation to planet formation.
There were previous indications that there is a strong positive relationship between metallicity and the presence of planets, where high metallicity would particularly lead to an overdose of (large) planets and/or large planets in the inner system. At the other end of the spectrum very low metallicity would lead to a failed planetary system with mainly dust and planetoids.
If indeed at least 2/3 and possibly a higher fraction of all (F, G, K) stars have planets and particularly large planets in the innermost system, how would that correllate with metallicity? Are there any ideas on that?
Spaceman: an alternative way for a giant planet acquiring an (near-) earthsized by capture may be a possible but very rare event, as suggested by the article cited by Rob Henry above. It requires a so-called “three-body gravitational encounter”.
This reminds me of the reason why secondary (non-original) binary stars, i.e. from different, single origins and captured in eachothers gravity also seem to be exceedingly rare.
Kurt9: your question about the reason for such commonness of gas/ice subgiants in inner systems and their apparent inward migration is indeed a very relevant one.
See also my previous post with question about metallicity: I have been wondering whether there is any correllation with metallicity.
My idea (for what it is worth) was that high metallicity (besides, to a lesser extent, stellar mass) leads to an overdose of (especially giant) planets, either also originally formed in the inner system and/or inward migration as a result of dragging and mutual perturbation.
However, with such an abundance of large planets in inner systems I begin wondering whether (high) metallicity can be a good explanation for this at all.
(I have just posed a similar question about metallicity on the systemic site).
Talking of migration, we don’t know that it didn’t happen in our solar system. One thing that comes out of simulations is that Jupiter-Saturn pairs where the outer planet is less massive tend to migrate outwards. This would allow for scenarios where Jupiter begins migrating inward, until Saturn catches up and reverses the migration. In fact, that turns out to be very helpful for explaining things like the low mass of Mars and various properties of the asteroid belt.
Spaceman, the size estimates that you are using for a habitable planet start by assuming both continental drift and strong magnet fields are necessary to a living planet. They continue by using models for drivers of these phenomena that have been built from the example of our own solar system. This process makes these over-dependant on current data from our own system. Two factors should give us pause for thought though.
If Mars (0.1 Earth masses) could have retained the above two properties for its first billion years or so, it seems that rock weathering temperature regulation might have allowed it to remain wet and warm for a long time in its current orbit.
If Mars was in Earth’s orbit there is a hint of evidence that it may have been crawling with life even without continental drift or a strong magnetic field (in the epoch before its atmosphere was ablated by the solar wind). To back such an extreme assertion, I will first note that it is very hard to explain the results of all Vikings biological experiments abiologically. I will continue by noting that the Martian seasonal wave of darkening would better fit a massive (endolithic) biosphere model for today’s Mars than the current windblown dust models that are used to explain this activity. Biological activity could be expected to be much higher if Mars was warm enough to support higher water activity.
Our current thinking may be correct but do not be blinkered into believing that it is based on particularly solid evidence.
Why does an exomoon have to be Earth-sized to be habitable? A moon roughly the size of Mercury could hold an atmosphere, look at Titan. And of course a moon this small could exist around even a saturn sized habitable planet.
Titan has its atmosphere because it is cryogenic cold. There is less leakage of the atmosphere into space. Mars, being closer in to the sun, lost its atmosphere eons ago. In the HZ, a planet really does need to be near Earth-mass in order to retain its atmosphere over the eons.
Seth Shostak (from SETI) has been crunching the numbers too he said
“If we crudely do the numbers, these early results from Kepler indicate that approximately 3 percent of all stars could boast a habitable planet” http://www.huffingtonpost.com/seth-shostak/a-bucketful-of-worlds_b_817921.html
Kurt9, are you implying that a Mars with an Earth-strength magnetic field would still undergo similar atmospheric loss, or are you just relying on it not having such a field. If your argument is the latter, then I think you would be surprised to find the extent that our current models for magnetic field generation from the cores of rocky planets has been built from data from our own solar system. In this second case your argument against Mars-sized biospheres thus subtly circular.
Does a moon have to have an atmosphere to be habitable? As far as I know we’re still looking for life in some crazy (I wouldn’t live there) places in our own solar system. How close would Jupiter have to be to the Sun to thaw out Europa or Ganymede just a bit?
Good question Bounty. The argument seems one of degree of habitability not habitability itself. Although Lovelock puts up a very good argument for an all or nothing biosphere, his is the minority view. The rest of us can easily imagine life just turning over on a place like Europa, but even here the difference with Titan is instructive.
Given the most imaginative scenarios under the thick ice model of Europa, one can only provide enough bioenergy for a few hundred tons of residual bacterial growth over that entire world each year. By contrast, Titan may be much further from the sun, but its lakes and surface continually soak up enough photo-generated hydrogen, that they could supports the equivalent metabolic energy to that used by an active troupe of 20 million humans, from this flow alone (here my working assumed that the hydrogen reduced formaldehyde – if it reduced acetylene it would be more).
If you are to imagine an ‘interesting’ ecology without an atmosphere you would probably have to give it a convenient translucent uv absorbing layer with water just below. Your idea of warming Europa just enough to turn a thick ice crust into a thin layer may well fit the bill here.
Option B might be to imagine a world very like Io, where volcanically generated flows, might bring surface generated high-energy compounds into that moons protected depths. The problem here is that most current exobiologists have difficulty imagining life as being consistent with the dearth of water implied in such cases.
Following on the most recent discussion, although we shouldn’t rule too much out yet right now when it comes to planets and life, it is likely that under the most adverse or marginal conditions mainly very primitive life can exist, equivalent to Prokaryota (bacteria, cyanobacteria) on earth.
That is to say: without a significant atmosphere and under extremely cold conditions. With regard to the latter, metabolism and evolution will inevitably be very slow. Ok, one might argue that internal heat could then provide the required energy. However, if (deep) soil heat is the main energy source, this will mainly promote the equivalent of soil bacteria, plus the fact that in a very small planet internal heat will decrease rapidly.
For fundamental and universal physical and biochemical reasons it is likely that (by far most, nearly all) higher life will require a few essential ingredients: carbon, liquid water, probably also molecular oxygen. For this reason and because higher life seems to take a very long time to evolve, a planet needs to have a very long-term ‘lifespan’ and stability, i.e. plate tectonics and an atmosphere, probably also a magnetic field induced by its liquid outer core. This in turn requires a sufficiently large mass planet.
While it may be true that a denser atmosphere might compensate for the lack of a magnetic field and keep a planet warm, this would also require a larger mass planet. A small (Mars like) planet closer to the sun (about Venus distance), as some have argued, would indeed stay warm longer than our Mars, but at the same time would also face quickly losing its water as a result of photodissociation and (part of) its atmosphere as a result of stronger solar wind, particularly in the youth stages of its sun.
Most researchers after Dole (1964) consider 0.93 – 0.95 AU the inner boundary of our HZ. Likewise, most researchers consider about 1/3 of earth mass the absolute lower limit for an earthlike planet in order to retain its atmosphere, its plate tectonics and possibly also its magnetic field.
Venus at Martian distance, that might have been something else.
Ronald, in regards to your implicit assertion that only primitive live evolves from harsh environments, why then the current consensus that no such higher life seems to have evolved on a ‘friendly’ Earth, but rather that it happened when our planet went through a series of Europa-like Snowball Earth episodes. Perhaps our biggest concern should be how energetic these ecologies could become, not their complexity.
On the other hand, your belief that the cold trap works very much better on planets that are more massive looks devastating for the prospects of a thriving biosphere on a Mars sized planet that has an Earth sized atmosphere, but is it true? I note that Earth’s gravity is also well below the level of that required to retain hydrogen, and that on a Mars-sized planet the atmosphere tails off slower with height and so should extend much further above their cloud decks. If this process is only a little faster on a Mars sized planet, its main effect should be a quicker build up of oxygen – more impetus for higher life to develop.
Ronald, you could be right but I would need to see your reference to know for certain. This is because, if this process was only slightly higher on Earth-like planets, it would just make Mars-like planets much better than ours at fostering higher life than heavier ones.
Rob, an attempt to reference:
– “The Heat History of the Earth”. Geolab. James Madison University
– Raymond, Sean N.; Quinn, Thomas; Lunine, Jonathan I. (January 2007). “High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability”. Astrobiology 7 (Preprint): 66.
Aand foremost:
– “Earth: A Borderline Planet for Life?”. Harvard-Smithsonian Center for Astrophysics. 2008.
The general idea is that a planet needs to be a minimum mass (of at least 1/3 earth, possibly even more), in order to:
1) Retain a sufficient atmosphere.
2) Retain plate tectonics, needed to replenish the atmosphere.
3) Retain a magnetic field, resulting from (convections in) a liquid outer core.
If we postulate that life is possible on worlds like Enceladus and Titan, then there is almost NO POINT in discussing the Kepler results. Life would be virtually universal anyhow. The only systems WITHOUT life, in this scenario, are those of short-lifetime massive stars.
I think what is of most interest is evaluating the stats and probabilities of what size planets exist in the HZ’s. Something I don’t think we have the data to do yet, at least for sun-like stars.
kzb: you are right, in that case the concept of HZ would have to be redefined. As for now, I would suggest that what we presently call the HZ, is (at least) the HZ for ‘higher’ life (multicellular, specialized organs).
Besides, as it looks, the HZ is more limited on the inside (the hot side) than on the outside (the cold side). As I mentioned above, most researchers nowadays consider 0.93 – 0.95 AU the inner boundary of our HZ, rather strictly delimited. On the outside the boundary seems to be much more fuzzy, researcher’s estimates varying from 1.2 AU via the most commonly mentioned 1.5 AU to even much more (>= 3 AU).
The main reason may be that insolation (and any diffuse radiation) diminishes with the square power of distance, i.e. increases as a square power of (inverse) distance as one approaches the sun or any star. This probably results in some absolute solar/stellar insolation threshold beyond which almost any present water evaporates and gets photodissociated in early stages of a star’s and planet’s existence. Whereas on the outside of the HZ much more depends on the geological and atmospheric conditions of the planet itself.
Interestingly, my impression from various publications (e.g. “Age and mass of solar twins constrained by lithium abundance”; J.D. do Nascimento et al., 2009) is that something similar is the case with ‘habitable’ stars: solar type stars that are suitable for terrestrial planets with (higher) life seem to be more and sharper limited on the hot side (i.e. earlier spectral types) than on the cool side (i.e. later spectral types). The ‘hottest/brightest’ habitable sunlike stars are probably around F9/G0 (roughly corresponding to 1.05 – 1.1 solar mass, 1.25 – 1.3 solar lum, already on the high side). Beyond this, the star will emit a lot of aggressive radiation (particularly lots of hard UV) and its stable lifespan will be severely limited.
On the cool side, it seems that habitable solar type stars can go on a long way, at least down to K0/K1, probably K2, …
So, if this is true, not only are we not situated nicely in the middle of our HZ but strongly toward the hot inside of it, but also our sun is not an optimal or ‘middle-class’ sunlike star but tends toward the hot end of the habitable solar type spectrum.
Ronald, thanks for the advise, and I shall investigate the works you mentioned, but so far I have found they inevitably seem excessively based on data from our own system – even if that basis is from earlier work. This is why I was far more interested in your possible implication that atmospheric water loss was so sever on small worlds. Since the theory of this mechanism can be worked without elaborate modelling containing many parameters that could have only been gleaned by investigating the nature of own system, that really does have the potential to rule out Mars sized worlds. The three main conclusion that you have given are almost certainly derived more speculatively and thus probably cannot.
Actually, on reflection Ronald, I would like to add further to some earlier remarks of yours, and to our anthropocentric nature. You felt that carbon was the best basis for complex molecules, and water the best solvent for it. I have previously examined the basis for these claims extensively and found them to be particularly well based. The puzzle then becomes, why has carbon such a high cosmic abundance, and why does there seem to be more water around than any other solvent. If we are right in these two earlier assertions, then our universe doesn’t just allow life it looks like it is made for it. That should warn us that something is wrong with our original assessment even on those two safer points.
Rob Henry: I agree with later post of 10 Febr. that our universe looks so particularly suited for life.
Why I am so convinced that (virtually) all life has to be carbon-based is that only carbon can form the long and complex macromolecules necessary for life’s processes. Silicon can also do something like that but to a much lesser extent than carbon, it pale sin comparison. Remarkable is the fact that on and in the earth silicon is much more abundant than carbon and yet all life on earth is carbon based, rather telling isn’t it?
With ref. to my earlier post of February 10, 2011 at 14:06 on the suitability and limits of solar type stars, maybe I should have elaborated a bit further, especially with regard to the (sharp) limitation of suitable ‘habstars’ on the hot (early spectral) side of the range.
What really matters for the habitability of planets, especially for higher life, is not just whether a (terrestrial) planet is presently within it’s mother star’s HZ, but whether is has been and will be within the HZ long enough for (higher) life to originate and develop, the so-called Continuously Habitable Zone (CHZ).
For our own sun and its CHZ and life on earth, it is known that first life (living cells, Prokaryotes) appeared when sun and earth were about 0.5 – 0.7 gy, that is when the earth itself had cooled down sufficiently, asteroid bombardment was not too heavy, the primordial atmosphere and crust had formed. And the sun was no longer emitting lethally high doses of UV.
Higher life, i.e. complex multi-celled with specialized organs arose much later, when sun and earth were about 4 gy, or about 0.6 gy ago. Probably because the oceans and crust were saturated woth O2, allowing atmospheric O2 levels to rise sufficiently and a protective ozone layer to form and land to be colonized.
Although our sample is limited to only one, there seems to be a kind of rule and timeline here that primitive life will arise when planetary and stellar conditions are even marginally permitting, within a few hundred million years, but complex life only when conditions are much more favorable and taking some 4 gy.
At the other, future, end of the timeline, it is known that, although the sun will remain on the main sequence for another 5 gy orso, it will already get too hot for higher organisms (or all organisms other than thermophiles) on earth in another 0.5 or 0.6 gy (and for all life in about 1 gy).
In other words, although the earth will have been in the CHZ of our sun for 5 gy, conditions for higher life on earth will only be favorable for a total of some 1 gy at the most. This we could call our window of opportunity for higher life. If this is typical then a planet needs to be in its sunlike star’s CHZ for at least 4 gy, in order for higher life to develop and prosper.
It is also known that the bigger (mass) and brighter (luminosity) a star, the shorter its lifespan and, more importantly, the shorter its ‘habitable lifespan’, or rather, that of any planet in its CHZ, simply because the CHZ of bigger/brighter stars (i.e. earlier spectral type), though being wider, moves outward more quickly (luminosity being roughly a third power of mass and lifespan being related to the ratio of mass/luminosity).
There comes a point where this habitable lifespan, or the existence of a planet in its CHZ, becomes too short for complex life to develop, in other words this window of opportunity decreases to virtually nil.
I came to the conclusion that the point where habitable lifespan becomes too short is about at 1.1 solar mass and 1.3 solar lum. Gleaning through nearby solar type stars of earlier spectral types and comparing mass and luminosity, this generally corresponds with spectral type F9/G0. I guesstimated that the habitable lifespan of those varies roughly from 3.8 – 4.5 gy, the average being close to 4 gy.
Of course, this is all still rather speculative, but if true then it indicates that around F9 would be the high end limit for a habstar suitable for higher life.
The Kepler findings to date indicate that about 15%-20% of stars have planets within .3 AU including planets of Earth, Super-Earth, Neptune, and Jovian size. This surprising result (very different from our solar system) can be accounted for based on migration of such planets from further out in the disk . But why did such migration occur in these systems and not ours (or most other stars)? The likely answer is photoevaporation from surrounding O and/or B stars in the cluster where young stars are forming. In the hellish enviornment of the Orion nebula, such photoevaporation is affecting 90% of more of all young stars and , apparently, most newly evolving stars (including our own sun) are born in such enviornments.
When a star is subject to such photoevaporation, the direction of flow of nebular material (surrounding the young star) is out instead of in and the outer part of the disk is truncated. As a result, Jupiter and Saturn did not fall into the sun (or fall into tight orbits around the sun). Furthermore, so much material was lost from the outer part of the disk that Uranus and Neptune only became ice giants instead of gas giants.
On the other hand, when a star is not subject to such photoevaporation, nebular material falls toward the newly evolving star resulting in the so-called “hot Jupiters”, “hot Neptunes”, and giant planets in the 1-2 AU range with extreme eccentricities (because such planets have engaged in a cosmic billiard game involving massive gravitational scattering).
As for the effect of metallicity on the star, there are strong indications that stars with higher metallicity have more Jupiters. On the other hand, Neptunes seem to form readily among stars with both high metal content and low metal content.
Therfore, I am hopeful that when Kepler gets results from .5 AU to 1.5 AU around solar -type stars, the presence of terrestrial planets (many of roughly earth-size and chemistry I hope) will be the rule rather the exception.
Ronald I note that near the end of your most recent post you write “of cause, all this is still rather speculative”. Might I also add that this is all rather brilliant work, and that problems only comes if we let that one aspect blind us to the other.
Otherwise there are only two points with which I might take issue. Firstly is there any analysis of the late heavy bombardment that would allow us some confidence that we can take 0.5 – 0.7 billion years as the typical time for a system to first settle down to allow life? Secondly, if you take the theory of evolution as fundamental to biology then the complexification of life can be explained but its origin is a exceedingly difficult to. Like so many other’s you have reversed these here, even if you are in good company.
Rob, I still didn’t reply to your latest question and particularly the issue “is there any analysis of the late heavy bombardment that would allow us some confidence that we can take 0.5 – 0.7 billion years as the typical time for a system to first settle down to allow life?”
Well, there seem to be a few related phenomena that spoil fun for the originating and early development of life;
Earth:
– very heavy bombardment by remaining leftovers of the early solar system;
– time needed for cooling and solidification of the crust;
– formation of the oceans;
– formation of the primordial atmosphere;
Sun:
– ‘subduing’ of extreme early solar activity (solar flares, coronal mass ejections) and extreme variations (up to half of luminosity), emitting very large amounts of aggressive radiation, partic. UV and X-ray.
All this together sets a time-frame for the start and early evolution of life. I am not sure about the exact timeline, but I would tentatively suggest that it takes several hunderd million years (at least 0.2 gy, but probably 0.4 – 0.8 gy) for conditions on a terrestrial planet near a sunlike star to be sufficiently favorable for first life.
For higher life there is at leat another condition that sets a timeframe: the crust and oceans first have to be sufficiently saturated with oxygen, so that O2 can build up in the atmosphere and an ozone layer can form.
Orrery of Kepler’s Exoplanets
Here’s a terrific visualization of all the multiple-planet systems discovered by the Kepler spacecraft as of February 2, 2011. The planets’ orbits go through the entire 3.5 year mission.
http://www.universetoday.com/83468/orrery-of-keplers-exoplanets/
Oops! Hopes for alien Earth go poof
By Alan Boyle
I should have known it was too good to be true: Last month, it looked as if a world known as KOI 326.01 was the best hope among the Kepler mission’s 1,235 candidates to be a second Earth.
It was thought to be a bit smaller than Earth, and even better, it was located in a “habitable zone.” That’s the area of space surrounding a star where water could plausibly exist in liquid form. Those two characteristics — smaller than Earth, and in the habitable zone — put KOI 326.01 in a class by itself.
No more, unfortunately. A fact-checker at Discover magazine, Mara Grunbaum, called up the Kepler team for more information about the planet, presumably because it was going to be featured in a future issue. Members of the team, including San Jose State University’s Natalie Batalha, double-checked their figures and determined that the planet candidate is actually somewhat warmer and much larger than originally estimated.
Full article here:
http://cosmiclog.msnbc.msn.com/_news/2011/03/09/6229949-oops-hopes-for-alien-earth-go-poof?email=html
A visualization of every star and exoplanet found by Kepler in one shot:
http://www.universetoday.com/84470/amazing-image-keplers-transiting-exoplanets/