STScI: Milky Way Contains at Least 100 Billion Planets

[bystander]

The Milky Way Contains at Least 100 Billion Planets According to Survey
NASA STScI | HubbleSite | 2012 Jan 11

[url=http://hubblesite.org/newscenter/archive/releases/2012/07/image/a/][b][i]Planets Everywhere[/i][/b][/url] [i](Credit: NASA, ESA, and M. Kornmesser (ESO))[/i]

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Our Milky Way galaxy contains a minimum of 100 billion planets according to a detailed statistical study based on the detection of three extrasolar planets by an observational technique called microlensing.

Kailash Sahu, of the Space Telescope Science Institute in Baltimore, Md., is part of an international team reporting today that our galaxy contains a minimum of one planet for every star on average. This means that there is likely to be a minimum of 1,500 planets within just 50 light-years of Earth.

The results are based on observations taken over six years by the PLANET (Probing Lensing Anomalies NETwork) collaboration, which Sahu co-founded in 1995. The study concludes that there are far more Earth-sized planets than bloated Jupiter-sized worlds. This is based on calibrating a planetary mass function that shows the number of planets increases for lower mass worlds. A rough estimate from this survey would point to the existence of more than 10 billion terrestrial planets across our galaxy.

The results are being published in the January 12 issue of the British science journal Nature.

The team's conclusions are gleaned from a planet search technique called microlensing. The technique takes advantage of the random motions of stars, which are generally too small to be noticed. If one star passes precisely in front of another star, the gravity of the foreground star bends the light from the background star.

This means that the foreground star acts like a giant lens amplifying the light from the background star. A planetary companion around the foreground star can produce additional brightening of the background star. This additional brightening reveals the planet, which is otherwise too faint to be seen by telescopes.

The higher the mass of the "lensing" star, the longer is the duration of the microlensing event. Typical microlensing events due to a star last about a month. But the extra brightening due to a planet typically lasts a few hours to a couple of days.

Using the microlensing technique, astronomers can determine a planet's mass. This method, however, does not reveal any clues about the world's composition.

Unlike other prominent planet-detection techniques, which measure the shadows of planets passing in front of their stars (transit) or measure the wobble of a star due to the gravitational tug of a planet (radial velocity and astrometry), the gravitational-lensing technique is unbiased in the selection of the host star.

The other techniques work best for finding planets close to their stars with short orbital periods. But microlensing can detect a planet that is as far from its star as Saturn is from our Sun, or as close as Mercury is to our Sun. The technique is also sensitive to detecting planets as small as Mercury.

Wide-field survey campaigns such as OGLE (Optical Gravitational Lensing Experiment) and MOA (Microlensing Observations in Astrophysics) cover millions of stars every clear night in order to identify and alert stellar microlensing events as early as possible. Follow-up collaborations, such as PLANET, monitor selected candidates more frequently, 24 hours a day, using a round-the-world network of telescopes.

Of the approximately 40 microlensing events closely monitored, three showed evidence for exoplanets. Using a statistical analysis, the team found that one in six stars hosts a Jupiter-mass planet. What's more, half of the stars have Neptune-mass planets, and two-thirds of the stars have Earth-mass planets. Therefore, low-mass planets are more abundant than their massive counterparts.

"This means, statistically, every star in the galaxy should have at least one planet, and probably more," said Sahu.

"Results from the three main techniques of planet detection are rapidly converging to a common result: Not only are planets common in the galaxy, but there are more small planets than large ones," said Stephen Kane, a co-author from NASA's Exoplanet Science Institute at the California Institute of Technology, Pasadena, Calif. "This is encouraging news for investigations into habitable planets."

These results are independent from a gravitational-lens survey led by Takahiro Sumi of Osaka University in Japan, which estimates there are hundreds of billions of planets with orbits larger than Saturn's orbit, or are free-floating throughout the galaxy.

Planet Population is Plentiful
European Southern Observatory | 2012 Jan 11

Planets around stars are the rule rather than the exception

An international team, including three astronomers from the European Southern Observatory (ESO), has used the technique of gravitational microlensing to measure how common planets are in the Milky Way. After a six-year search that surveyed millions of stars, the team concludes that planets around stars are the rule rather than the exception. The results will appear in the journal Nature on 12 January 2012.

Over the past 16 years, astronomers have detected more than 700 confirmed exoplanets ^[1] and have started to probe the spectra (eso1002) and atmospheres (eso1047) of these worlds. While studying the properties of individual exoplanets is undeniably valuable, a much more basic question remains: how commonplace are planets in the Milky Way?

Most currently known exoplanets were found either by detecting the effect of the gravitational pull of the planet on its host star or by catching the planet as it passes in front of its star and slightly dims it. Both of these techniques are much more sensitive to planets that are either massive or close to their stars, or both, and many planets will be missed.

An international team of astronomers has searched for exoplanets using a totally different method — gravitational microlensing — that can detect planets over a wide range of mass and those that lie much further from their stars.

Arnaud Cassan (Institut dʼAstrophysique de Paris), lead author of the Nature paper, explains: "We have searched for evidence for exoplanets in six years of microlensing observations. Remarkably, these data show that planets are more common than stars in our galaxy. We also found that lighter planets, such as super-Earths or cool Neptunes, must be more common than heavier ones."

The astronomers used observations, supplied by the PLANET ^[2] and OGLE ^[3] teams, in which exoplanets are detected by the way that the gravitational field of their host stars, combined with that of possible planets, acts like a lens, magnifying the light of a background star. If the star that acts as a lens has a planet in orbit around it, the planet can make a detectable contribution to the brightening effect on the background star.

Jean-Philippe Beaulieu (Institut d'Astrophysique de Paris), leader of the PLANET collaboration adds: "The PLANET collaboration was established to follow up promising microlensing events with a round-the-world network of telescopes located in the southern hemisphere, from Australia and South Africa to Chile. ESO telescopes contributed greatly to these surveys.”

Microlensing is a very powerful tool, with the potential to detect exoplanets that could never be found any other way. But a very rare chance alignment of a background and lensing star is required for a microlensing event to be seen at all. And, to spot a planet during an event, an additional chance alignment of the planet’s orbit is also needed.

Although for these reasons finding a planet by microlensing is far from an easy task, in the six year's worth of microlensing data used in the analysis, three exoplanets were actually detected in the PLANET and OGLE searches: a super-Earth ^[4], and planets with masses comparable to Neptune and Jupiter. By microlensing standards, this is an impressive haul. In detecting three planets, either the astronomers were incredibly lucky and had hit the jackpot despite huge odds against them, or planets are so abundant in the Milky Way that it was almost inevitable ^[5].

The astronomers then combined information about the three positive exoplanet detections with seven additional detections from earlier work, as well as the huge numbers of non-detections in the six year's worth of data — non-detections are just as important for the statistical analysis and are much more numerous. The conclusion was that one in six of the stars studied hosts a planet of similar mass to Jupiter, half have Neptune-mass planets and two thirds have super-Earths. The survey was sensitive to planets between 75 million kilometres and 1.5 billion kilometres from their stars (in the Solar System this range would include all the planets from Venus to Saturn) and with masses ranging from five times the Earth up to ten times Jupiter.

Combining the results suggests strongly that the average number of planets around a star is greater than one. They are the rule rather than the exception.

“We used to think that the Earth might be unique in our galaxy. But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way,” concludes Daniel Kubas, co-lead author of the paper.

1. Notes
2. The Kepler mission is discovering huge numbers of “candidate exoplanets” that are not included in this number.
3. Probing Lensing Anomalies NETwork. More than half of the data from the PLANET survey used in this study come from the Danish 1.54-metre telescope at ESO's La Silla Observatory.
4. Optical Gravitational Lensing Experiment.
5. A super-Earth has a mass between two and ten times that of the Earth. So far 12 microlensing planets have been published in total, using various observational strategies.
6. The astronomers surveyed millions of stars looking for microlensing events. Only 3247 such events in 2002-2007 were spotted as the precise alignment needed is very unlikely. Statistical results were inferred from detections and non-detections on a representative subset of 440 light curves.

One or more bound planets per Milky Way star from microlensing observations - A. Cassan et al

• Nature 481 167 (12 Jan 2012) DOI: 10.1038/nature10684 (pdf)

A wealth of habitable planets in the Milky Way
Niels Bohr Institute | University of Copenhagen | 2012 Jan 11

Study Shows Our Galaxy Has at Least 100 Billion Planets
NASA JPL-Caltech | 2012 Jan 11

Planets around stars are the rule rather than the exception
Lawrence Livermore National Laboratory | 2012 Jan 11

Milky Way Crammed With 100 Billion Alien Worlds?
Discovery News | Ian O'Neill | 2012 Jan 11

Microlensing Study Says Every Star in the Milky Way has Planets
Universe Today | Nancy Atkinson | 2012 Jan 11

Exoplanet news part 3: There may be hundreds of *billions* of planets in our galaxy!
Discover Blogs | Bad Astronomy | 2012 Jan 13

[orin stepanek]

That's more planets than earth people!

[Doum]

That's more planets than earth people!

Yea but it is at least 14 to 15 planets per people. I'm rich.

[geckzilla]

Sounds like a free planet giveaway on the Oprah show. You get a planet! And you get a planet!

[bystander]

You all go ahead and go out and claim your planets. I'll stay here and keep this one.

Be sure to write!

[Flase]

Of course it will be difficult to visit one of these places in a hundred lifetimes, but what say you do and there are natives? They might not like you. They might be scared of these ape-creatures from the skies...

[Doum]

Wow, even more planet.

http://www.sciencedaily.com/releases/20 ... 090937.htm

[Céline Richard]

As we still didn't find out at least another one habitable planet, we should learn to love each others more than what we do. "We stand together, taken away on the same planet, as the crew of a same ship" (Saint-Exupéry).
I am sure it is possible, but i wonder how long the road is. Maybe we have to suffer from our mistakes, our failures to make a better world -with more love- because it would be more worthy to live in a wonderland if we make it up, instead of applying a recipe.
Kiss from France

[neufer]

A rough estimate from this survey would point to the existence of more than 10 billion terrestrial planets across our galaxy.

The Drake equation states that:



where: N = the number of civilizations in our galaxy with which communication might be possible;

and
[list]
R* = the average rate of star formation per year in our galaxy
f_p = the fraction of those stars that have planets
n_e = the average number of planets that can potentially support life per star that has planets
f_ℓ = the fraction of the above that actually go on to develop life at some point
f_i = the fraction of the above that actually go on to develop intelligent life
f_c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space[/list]
Considerable disagreement on the values of most of these parameters exists,
but the values used by Drake and his colleagues in 1961 were:[dubious – discuss]

[list] R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)
f_p = 0.5 (half of all stars formed will have planets)
n_e = 2 (stars with planets will have 2 planets capable of developing life)
f_l = 1 (50% of these planets will develop life)
f_i = 0.01 (1% of which will be intelligent life)
f_c = 0.01 (1% of which will be able to communicate)
L = 10,000 years (which will last 10,000 years)[/list]
Drake's values give N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10.>>

[Céline Richard]

• R* = the average rate of star formation per year in our galaxy
  f_p = the fraction of those stars that have planets
  n_e = the average number of planets that can potentially support life per star that has planets
  f_ℓ = the fraction of the above that actually go on to develop life at some point
  f_i = the fraction of the above that actually go on to develop intelligent life
  f_c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
  L = the length of time for which such civilizations release detectable signals into space

Considerable disagreement on the values of most of these parameters exists,
but the values used by Drake and his colleagues in 1961 were:[dubious – discuss]

• R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)
  f_p = 0.5 (half of all stars formed will have planets)
  n_e = 2 (stars with planets will have 2 planets capable of developing life)
  f_l = 1 (50% of these planets will develop life)
  f_i = 0.01 (1% of which will be intelligent life)
  f_c = 0.01 (1% of which will be able to communicate)
  L = 10,000 years (which will last 10,000 years)

Drake's values give N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10.>>[/size]

Please, let's imagine that the average rate of star formation per year in most of the galaxies is far different from the one of our Galaxy. Moreover, let's suppose that the fraction of those stars that have planets depends on the position of each galaxy within the cosmos. Indeed, inside the ocean for instance, life develops more near the surface (closer to the sunlight, at lower pressures) than in the abyss in darkness; if the proliferation of life doesn't seem to be isotropic in the oceans, why would it be easier in the cosmos? If a civilization lasts 10,000 years, it might become able to travel to other planets and send messages from those planets (maybe sending messages from a planet which is not the native planet would be easier, it could depend on the technology made it up by each civilization). So how many spots should the equation consider to take into account that a same civilization could send messages from different planets?
Anyway, i like the answer given by the Drake equation : we are not likely to be alone in the Universe.
I hope that one day, if we get some scientific clues that another intelligent life had sent signals toward us, our scientists would be able to decipher those messages, to understand what it means (mathematics might be a universal language to describe the world, but the symbols we can use for it are infinite). I also hope the time we need to understand the message will be short enough to enable us to answer, and the other civilization to get our answer and understand it.
I wonder what would be the most important to tell and how. It might be complicated to send books (anatomy, literature, poems...), musics, pictures of the earth (or paintings) to express our beliefs and feelings, and to describe our knowledge.

[Chris Peterson]

Please, let's imagine that the average rate of star formation per year in most of the galaxies is far different from the one of our Galaxy.

The suggested value of 10 seems low for our own galaxy. And our galaxy is probably fairly typical in this respect.

Moreover, let's suppose that the fraction of those stars that have planets depends on the position of each galaxy within the cosmos.

I would say this is most unlikely. It is fundamental to nearly all modern cosmological theories that there is no preferred place in the cosmos- conditions are the same everywhere. Certainly, we don't see much difference in the nature of galaxies with position. At most, you might expect galaxies in somewhat denser clusters to have a higher rate of star formation due to collisions... but those same dynamics might also reduce the likelihood of stable planetary systems forming.

Indeed, inside the ocean for instance, life develops more near the surface (closer to the sunlight, at lower pressures) than in the abyss in darkness; if the proliferation of life doesn't seem to be isotropic in the oceans, why would it be easier in the cosmos?

Because the oceans are not isotropic; the cosmos apparently is.

If a civilization lasts 10,000 years, it might become able to travel to other planets and send messages from those planets (maybe sending messages from a planet which is not the native planet would be easier, it could depend on the technology made it up by each civilization). So how many spots should the equation consider to take into account that a same civilization could send messages from different planets?

It seems likely that if a civilization traveled to another star, the daughter civilization would be pretty much on its own, and begin diverging both culturally and genetically from its parent. Soon enough, they could be considered different civilizations, even if they had a common origin.

[Ann]

I'm unimpressed with the Drake Equation. We can make an educated guess - a very educated guess - about how many stars there are in the Milky Way, and we are learning more and more about how many planets there may be in the Milky Way.

But how can we say anything about what it takes for life to emerge on other planets? So far, we have not found any credible evidence that life has ever existed anywhere else but here on Earth. Scientists don't agree on the processes and the conditions here on Earth that allowed life to come into existence. So how can we say that as long as a planet is orbiting another star inside the "habitable zone" of that star, it has a 50% chance of actually "cooking up life" on its surface or below its surface? Or that it has a 25% chance of doing so? Or a 75% chance? As long as we don't know how life arises out of non-life, and as long as we have an example of one where it has happened, how can we estimate the chances of it happening elsewhere?

Let's assume that we are going to find evidence that simple, bacteria-like life forms emerge quickly and easily as soon as there is a supply of a suitable solvent and a source of energy. If we were to find such evidence, for example by finding non-Earthly bacteria on other planets in our solar system, then we would suddenly have good reasons to assume that bacteria-like life forms are abundant in the Milky Way and in the universe. So far, however, we lack such evidence.

And how do we know what it takes for bacteria to evolve into a technological civilization? When we have an example of one of where it happened, and where it took about three billion years (or more) for it to happen? Are we to assume that as long as bacteria establish themselves on a planet, then some of those bacteria have a 0.0001% chance of having evolved into a technological civilization after three billion years? So that if we can find a million planets with bacteria, then we can reasonably assume that after three billion years one of them will be able to send messages to us to say hello?

And what do we know about how long a technological civilization like ours will last, seeing that we are using up our resources faster than they can be replenished so that we can maintain our technological civilization?

The Drake Equation is fun, and it gives food for thought. But in my opinion, we just don't know even nearly enough about how life comes into existence, how it typically evolves or how long a technological civilization typically lasts to use the Drake Equation to make predictions or estimates of any scientific value whatsoever.

Ann

[rstevenson]

Hi Ann,

You've aptly described the difference between a scientific theory -- something that is well-supported by data and useful in accurately describing some aspect of reality -- and a scientific hypothesis -- "This might be true. Let's see if we can find some data to either support it or refute it."

The Drake equation is a way to formulate an hypothesis. One person may use it and conclude there is not even one technological civilization besides ours in the galaxy right now. Another person may conclude there are 5000 of them. But as you say, at that point we stop, lacking any data to advance one or the other hypothesis in the direction of becoming a theory.

We've recently been finding a lot of data which seems to tell us how many planets there are likely to be in the galaxy, and how many of those might be in the liquid-water zone. So we're now able to make better guesses for a couple of factors in the Drake equation. And there is other research that shows several of the building blocks of life, crude and simple though they may be, are apparently ubiquitous through large regions of the galaxy. So it seems to me likely that there is at least pond slime almost everywhere it can find a niche. But again, as you say, we've no data on how often such stuff evolves into something we could talk to.

But it is fun to do the equation once in a while. There is a good Drake Equation calculator here, on the PBS Nova site. I just got 4854! What's your Drake number?

Rob

[Chris Peterson]

I'm unimpressed with the Drake Equation...

I can't see anything wrong with the equation at all. It is a tool for identifying the factors necessary to estimate the number of technological civilizations in a galaxy (or life in general, when a subset of the equation is used). The equation can focus thinking in two ways: it encourages theorists to consider whether there are any important terms missing, and it encourages observations which can narrow down the range of values for different terms.

Certainly, there is currently a huge uncertainty in most of the terms, so no useful number can emerge. But I do think that most of the terms can ultimately be restricted to usefully narrow ranges... so the equation itself remains valid and useful. This is really no different than how a number of important scientific "theories" are handled- Big Bang theories, global climate theories, and others- where the underlying theory (or really, model based on multiple theories) is considered quite solid, but important terms remain poorly constrained. This doesn't make the theory less valid, it simply makes the conclusions uncertain... but not ultimately unknowable.

[Ann]

I agree, Chris, there is nothing wrong with the Drake Equation itself. What I object to is when people use the Drake Equation to quantify the number of technological civilizations that can be expected to exist in our galaxy. In my opinion, we just don't have sufficient knowledge about how life arises and evolves to make any sort of educated guesses about the number of technological civilizations that might exist in the Milky Way.

Ann

[geckzilla]

Can we ever have enough knowledge about anything (well, almost anything... no, Art, no!)? Surely there's no harm in making an educated guess, anyway. I don't follow your logic at all, Ann.

[Ann]

Can we ever have enough knowledge about anything (well, almost anything... no, Art, no!)? Surely there's no harm in making an educated guess, anyway. I don't follow your logic at all, Ann.

When scientists feel they have figured out how life actually arose here on Earth, what actual step-by-step processes were involved, what specific conditions were necessary for these processes to take place, then I think we have a sufficiently good knowledge to make an educated guess about how many other planets in the Milky Way can support life. Alternatively, fail to figure out the actual processes and conditions during which life arises, but prove that life has come into existence on other planets, and again it will be possible to make an educated guess about life in other solar systems.

But as long as we have an example of one of life on a planet in the universe, and as long as scientists haven't figured out how that life came into being in the first place, then I don't think we can talk about "educated" guesses at all when it comes to the prevalence of life in the universe. All you have is guesses.

Perhaps there are a hundred million other technological civilizations in the Milky Way? Perhaps we are the only ones in the entire Local Group? For now, both these guesses are equally "educated".

Ann

[Chris Peterson]

When scientists feel they have figured out how life actually arose here on Earth, what actual step-by-step processes were involved, what specific conditions were necessary for these processes to take place, then I think we have a sufficiently good knowledge to make an educated guess about how many other planets in the Milky Way can support life.

I think that there are some very good ideas about how life developed, as well as supporting experimental evidence. Enough that I think it is not unreasonable to argue that life is very common in the Universe. Where things get weaker, I think, is the step from simple life (algae and bacteria) to complex, multicellular life. The latter is only a sliver in the timeline of life on this planet, and how that transition occurs remains very uncertain... and therefore makes it difficult to speculate on the ubiquity of complex life in the Universe.

[Céline Richard]

About 100 amino acids were found in the Murchison meteorit, fallen in 1969, in Australia. Among those amino acids, it seems that about 20 exist in cells. The Murchison meteorit derives from traces of asteroids, present at the time of the birth of the solar system 4,55 billion years ago.
I know that amino acids are not cells, but life on Earth needs amino acids.
Some organic molecules were found in comets, although it is only a step towards life, not life itself -not an alive cell.

It seems comets and asteroids can send to very different places in the cosmos some ingredients for life.
However, the development of life depends on environmental conditions too. If the Universe is infinite, i assume the number of environments likely to host life should be more than one.