APOD: Habitable Worlds (2014 Mar 03)

[APOD Robot]

Habitable Worlds

Explanation: Is Earth the only known world that can support life? In an effort to find life-habitable worlds outside our Solar System, stars similar to our Sun are being monitored for slight light decreases that indicate eclipsing planets. Many previously-unknown planets are being found, including over 700 worlds recently uncovered by NASA's Kepler satellite. Depicted above in artist's illustrations are twelve extrasolar planets that orbit in the habitable zones of their parent stars. These exoplanets have the right temperature for water to be a liquid on their surfaces, and so water-based life on Earth might be able to survive on them. Although technology cannot yet detect resident life, finding habitable exoplanets is a step that helps humanity to better understand its place in the cosmos.

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[somebodyshort]

Have the stars in the neighbourhood associated with the WOW signal been scanned?

[PacRim Jim]

Dibs on the Planet of the Three-breasted Beer-brewing Nymphomaniacs

[PhilT]

Since we are using light dipping to find these exoplanets it seems to me that methodology only identifies planetary systems which orbit in the same plane as our Earth lies relative to the star under study, or at least very close to that plane. At the distance we are from those stars,,,, that's a very thin plane.

The corollary to this is that we don't detect 99.99999....% of exoplanets - and thus a theory I've had for years - virtually EVERY star has planets, we just don't yet have a reliable way to detect them. Detecting a stars wobble would find more but then we wouldn't know how big the planet was or how near to the star.

Am I talking rubbish ? have I missed something ?

[somebodyshort]

@ PhilT
Bingo. Kepler can't find any planet that doesn't cross the stars face. Identifying a wobble with multiple planets is near impossible. Imagine even our own system how would the Sun wobble. I'm not even sure they could identify our outer planets because of the long "years". As an example: how often would Jupiter cross the stars face.

[jimlesire@cox.net]

"The right temperature" is one factor determining if water will be liquid, solid or gas, but so is pressure. Granted that is a more difficult parameter to determine, but since gravity plays a large part in the density of a planet's atmosphere, could it not be included?

[Perk Cartel]

Would it be fun to see on the various charts of phz candidates how far away are they?

[Nitpicker]

It must be very difficult to analyse such small signals within much larger signals. Does anyone know what the margins of error are in these findings? How many eclipses are typically observed per exoplanet candidate before confirmation? How accurate are the exoplanet semi-major axes and temperatures? How accurate are the exoplanet sizes, masses and surface gravity estimations? How accurate are the derived exoplanet compositions? I'm not doubting anyone, but there must be a some level of uncertainty within the numbers.

But perhaps the bigger question is: assuming a whole bunch of "Warm Terrans" are eventually confirmed, what do we do about it? How could we study them in more detail, given that their very existence is only just detectable, and a probe would seem out of the question? Do we simply blast them with high-energy encoded radio messages, and see if anything responds?

(Sorry, that's a lot of questions.)

[mollwollfumble]

I've been analysing the orbits of Kepler planets. There are three main situations in which a so-called "habitable" Kepler planet may not be habitable at all.

1. If the orbit is elliptical. So far as I can tell, no-one has checked these Kepler planets to see if the orbit is elliptical. By far the majority of binary stars found by Kepler are known to be in elliptical orbits. It is possible to determine statistically the probability that a Kepler planet is in an elliptical orbit. I did this and found that by far the majority of Kepler planets are also in elliptical orbits. An elliptical orbit would take a planet out of the habitable zone.

2. Kepler is only picking up planets in very close orbits - by far the majority of planets found are closer to the parent star than Mercury is to the Sun. In order for a Kepler planet to be in the "habitable zone" the parent star would have to be much fainter than the Sun, a dwarf. This has problems because the smaller the luminosity the narrower the "habitable zone". And this causes extra problems with ellipticity of orbit.

3. I found in analysing Kepler data that stray light from background sources was a much greater problem than had previously been estimated. The majority of objects that at first appeared to be a planet or brown dwarfs were actually faint binary stars. Most background light contamination can't be removed by using the standard off-centre light source method. A conclusion is that many if not most of the planets found by Kepler are larger than they appear to be. A planet thought to be Earth-size for instance may be more like the size of Neptune.

[mollwollfumble]

Have the stars in the neighbourhood associated with the WOW signal been scanned?

Many times. Also, it doesn't seem to be widely known but one of their publications shows that SETI has picked up hundreds if not millions of other signals similar to but stronger than the WOW! signal. Some few of these are well outside the statistical limits expected from natural sources and are also strongly suspected not to be man-made. These signals, being just narrowband pulses of power, carry no useful information and are not associated with any known star. Direct searches of more than 1000 nearby stars by SETI by the Parkes radio telescope and elsewhere have found nothing that I've seen published.

[mollwollfumble]

It must be very difficult to analyse such small signals within much larger signals. Does anyone know what the margins of error are in these findings? How many eclipses are typically observed per exoplanet candidate before confirmation? How accurate are the exoplanet semi-major axes and temperatures? How accurate are the exoplanet sizes, masses and surface gravity estimations? How accurate are the derived exoplanet compositions? I'm not doubting anyone, but there must be a some level of uncertainty within the numbers.

I can answer some of those.

> How many eclipses are typically observed per exoplanet candidate before confirmation?
For a typical Earth-size Kepler planet a hundred or so eclipses are analysed. The committee doesn't accept any planet into its list of planetary candidates until a minimum of three planetary eclipses are observed.

> How accurate are the exoplanet semi-major axes?
Very good. An acceptable error is about 0.01%. I personally spotted one possibility with an inconsistency of about 0.1% in semi-major axis - enough to eliminate it as a possible planet.

> temperatures?
On the dubious assumptions of circular orbit and ignoring any effect of a planetary atmosphere, temperatures are calculated directly from the stellar luminosity and semi-major axis. The results are pretty accurate. There is probably a table accessible of the web that gives estimated errors.

> How accurate are the exoplanet sizes?
Sizes from Kepler data are based on the time that the light curve dips from maximum to minimum brightness, with a correction based on how far the planet is out of alignment with the equator, which in turn comes from the total transit time. From what I've seen, exoplanet sizes are can be in error by up to about 20%, but I'd guess 5% error is more typical.

> masses and surface gravity estimations?
For Kepler data, these aren't particularly good. To get some idea, think three times the percentage error in size for Earth- to Neptune-size planets. For Saturn- to Jupiter-sized planets the mass can easily be wrong by a factor of 100 due to the problem that adding mass to Jupiter could cause it to shrink rather than grow because of the extra gravity compressing the gas. The mass-size relationship of Jupiter-sized planets becomes very sensitive to both planet age and composition.

> How accurate are the derived exoplanet compositions?
This is all modelled rather than measured, based on an assumed temperature at the time of planet formation. Migration of a planet inwards or outwards with age could change this considerably. The exception would be for the nearest bright stars where it is sometimes possible to measure the spectrum or thickness of the planet's atmosphere.

[Nitpicker]

It must be very difficult to analyse such small signals within much larger signals. Does anyone know what the margins of error are in these findings? How many eclipses are typically observed per exoplanet candidate before confirmation? How accurate are the exoplanet semi-major axes and temperatures? How accurate are the exoplanet sizes, masses and surface gravity estimations? How accurate are the derived exoplanet compositions? I'm not doubting anyone, but there must be a some level of uncertainty within the numbers.

I can answer some of those.

Thank you. Very informative. Sounds more accurate than I imagined. But you've noted the limitations nicely, too.

[Roger97]

I just would like to point out that there are two false statements in today's comments: “These exoplanets have the right temperature for water to be a liquid on their surfaces, and so water-based life on Earth might be able to survive on them.” We simply don't know that either statement is true to fact. For instance, an exoplanet may easily have a thick atmosphere like Venus, which raises the surface temperature far beyond the boiling point of water. The early Earth had a thick atmosphere, before the Mars-sized impactor event; many other terrestrial planets that are the approximate size of Earth may have similar, thick atmospheres, which may preclude the existence of liquid water on their surfaces. Also, even if the surface temperature did permit liquid water, a fair-sized ocean plus ongoing plate tectonics would be necessary to distribute sufficient nutrients for life to survive on that exoplanet for a long period of time, longer than a few months. And life requires not just SOME nutrients, but an abundant supply of MANY, specific nutrients in order to survive for any length of time. Small amounts of other nutrients are also necessary for life to thrive. And for advanced biological life to exist, there are many more factors that are required. (Some have estimated that there are hundreds of factors for advanced life to thrive!) We simply don’t if all of these factors or conditions currently coexist on any of these exoplanets.

[Chris Peterson]

It must be very difficult to analyse such small signals within much larger signals. Does anyone know what the margins of error are in these findings?

This paper covers details of instrumental noise. Detections of Earth-sized planets are just above the noise level, at 2.7 sigma. At least three orbits are required for a robust detection, which means a minimum of three years observation for a system that is similar to our own.

But perhaps the bigger question is: assuming a whole bunch of "Warm Terrans" are eventually confirmed, what do we do about it? How could we study them in more detail, given that their very existence is only just detectable, and a probe would seem out of the question? Do we simply blast them with high-energy encoded radio messages, and see if anything responds?

I don't think many scientists are interested in attempted communication. This research isn't about finding intelligent life. It's about understanding how systems like ours form, what their histories are, what kinds of conditions they support. The next step would likely be to develop (or improve) instruments that let us examine their atmospheres, and also their outer systems (looking for an Oort cloud, for instance). That's something that can be done from the ground with much larger telescopes. Kepler is a survey tool. What's at its noise level may be well above for other instruments.

[Chris Peterson]

Since we are using light dipping to find these exoplanets it seems to me that methodology only identifies planetary systems which orbit in the same plane as our Earth lies relative to the star under study, or at least very close to that plane. At the distance we are from those stars,,,, that's a very thin plane.

The corollary to this is that we don't detect 99.99999....% of exoplanets - and thus a theory I've had for years - virtually EVERY star has planets, we just don't yet have a reliable way to detect them. Detecting a stars wobble would find more but then we wouldn't know how big the planet was or how near to the star.

The actual value is 99.5%. That is, for a planet the size of the Earth at 1 AU from its star, we can detect 1/200 by occultation methods, based just on geometry.

The statistics produced by the Kepler mission support the idea that nearly every star has planets.

[BDanielMayfield]

Would it be fun to see on the various charts of phz candidates how far away are they?

I don't know how fun reading this is, but ...

In the order in which they are presented in today's APOD, the estimated distances in Light Years to the stars these 12 exoplanets orbit are:

Upper row: 22.7, 1,200, 11.9, 20.3, 22.7, 41.7
Lower row: 48.9, 1,064, 587, 1,200, 22.7, 20.3

Note that some of the above numbers repete, due to the fact that three of these exoplanets orbit the star Gliese 667C, and two each orbit Kepler-62 and Gliese 581.

Bruce

[Steve Dutch]

Discovering exoplanets in such profusion is thrilling, but there is no scientific value in illustrations like these. We know only the sizes of these planets, very roughly. We do not know their colors or surface characteristics, and presenting spurious details to the non-scientific public is misleading.

[Ann]

I'm curious about tomorrow's APOD. I thought planets were either habitable, i.e., (perhaps) able to support life, or inhabited, i.e., actually, definitely supporting life.

What's an inhabitable world? Mars?

Ann

[Chris Peterson]

I'm curious about tomorrow's APOD. I thought planets were either habitable, i.e., (perhaps) able to support life, or inhabited, i.e., actually, definitely supporting life.

What's an inhabitable world? Mars?

Having gotten some sense of the way the APOD editors' minds work, I'm leaning towards an image featuring the Earth.

[BDanielMayfield]

Before anyone books travel plans the climate of your destination should be considered. As we know pure water freezes at 0 and boils at 100 degrees C (at standard sea level atmospheric pressure). For comparison the Earth’s overall average surface temperature is 15 degrees C. Chemicals desolved in water widen the liquid water limits, and atmospheric pressure differences do so as well. Heat welling up from a planet’s interior and increased duration and/or incident angle of daylight can make areas of a cold planet warm, and decreased duration and/or incident angle of daylight can make areas of a hot planet cool.

In the order in which they are presented in today's APOD, the estimated mean temperatures in degrees C of these exoplanets (per Wikipedia) are:

Upper row: 23, -3, 70, -24, -34, 6
Lower row: 59, 0, 22, -65, -64, -92

This points out that as used in the exoplanet context “habitability” has a broad meaning, which shouldn’t be confused with “inhabitability,” at least by humans from this planet.

Bruce

[Chris Peterson]

In the order in which they are presented in today's APOD, the estimated mean temperatures in degrees C of these exoplanets (per Wikipedia) are:

Upper row: 23, -3, 70, -24, -34, 6
Lower row: 59, 0, 22, -65, -64, -92

I believe those are mean surface temperatures, not mean temperatures. Certainly, we have examples of lifeforms on Earth that can thrive at any of these, and we have a wider range of local temperatures on the Earth than this.

[Keyman]

Before anyone books travel plans ...

In a serendipitous happenstance, I am currently reading an Advabced Reader Copy of a soon-to-be-released collection of short stories by Ben Bova, titled "New Frontiers". In one of the stories ("A Country For Old Men") the first manned expedition to an extra-solar planet is headed for... Gliese 581. "The scoopship was named Sagan, after some minor twentieth century astronomer".

There is also, for fans of Casablanca (and if you're not, I'm never speaking to you again) a 'sequel' titled "We'll Always Have Paris".

[Boomer12k]

Yes, there IS life in outer space....US....it is just the way you look at things.

Very nice comparison of Exo-planets.
Back in the 70's I did a report on the number of possible planets...now it looks like I was right. Science always seems to start with a very conservative view of things. Then it gets closer and closer to the truth....and a looooong time in-between....

:---[===] *

[BDanielMayfield]

In the order in which they are presented in today's APOD, the estimated mean temperatures in degrees C of these exoplanets (per Wikipedia) are:

Upper row: 23, -3, 70, -24, -34, 6
Lower row: 59, 0, 22, -65, -64, -92

I believe those are mean surface temperatures, not mean temperatures. Certainly, we have examples of lifeforms on Earth that can thrive at any of these, and we have a wider range of local temperatures on the Earth than this.

Yes, surface is I believe what was meant too. Thanks for the added clarification Chris. And they would be educated guesses based on expected but uncertain albedos, atmospheres, etc. with considerable margins of error. Unknown rotation rates and axial tilt would also be of great import in controlling the range of temps any given site on any exoplanet might experience. But the numbers are mostly based on how the planet would be heated on average in it's stellar environment, and so these numbers can give us a rough idea of how Earth-like (or not) each case might be.

Bruce

[rstevenson]

... Science always seems to start with a very conservative view of things. Then it gets closer and closer to the truth....and a looooong time in-between....

I don't think that's right. Science doesn't take a conservative view of things, it has no view -- at least, not if "view" means having an opinion. Science simply states what it currently knows to be true, and will clarify that in as much detail as we wish to take in. But it won't go beyond that; speculation is not part of science and if science is to maintain its credibility, it must maintain that sense of propriety.

Of course it takes a long time (usually) to get closer to the truth. That's a good thing. The "truth" as it relates to our view of reality, is something that must be determined carefully, with considerable double-checking and fine-tuning, before it can be adequately communicated to anyone other than other scientists. Scientists walk a fine line. Too many public mistakes and credibility is lost. Endless careful checking and rigor mortis sets in.

I'm sure very few scientists thought, for example, that the Earth was unique and that we wouldn't find lots of planets around virtually every star we look at. But they couldn't say much about what they as individuals thought until they as scientists began to collect facts. Now they're doing that, and now (as both you and I and most sensible people expected) it looks like planets are ubiquitous.

Rob