Nearly All Sun-Like Stars Have Planetary Systems

by Nancy Atkinson on January 7, 2013

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A new analysis examined the frequencies of planets of different sizes based on findings from NASA's Kepler spacecraft, correcting for both incompleteness and false positives. The results show that one in six stars has an Earth-sized planet in a tight orbit. Credit: F. Fressin (CfA)

A new analysis examined the frequencies of planets of different sizes based on findings from NASA's Kepler spacecraft, correcting for both incompleteness and false positives. The results show that one in six stars has an Earth-sized planet in a tight orbit. Credit: F. Fressin (CfA)

The latest analysis of data from the Kepler planet-hunting spacecraft reveals that almost all stars have planets, and about 17 percent of stars have an Earth-sized planet in an orbit closer than Mercury. Since the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there, according to Francois Fressin of the Harvard-Smithsonian Center for Astrophysics (CfA), who presented new findings today in a press conference at the American Astronomical Society meeting in Long Beach, California. Moreover, he said, almost all Sun-like stars have planetary systems.

The holy grail of planet-hunting is finding a twin of Earth – a planet of about the same size and in the habitable zone around similar star. The odds of finding such a planet is becoming more likely Fressin said, as the latest analysis shows that small planets are equally common around small and large stars.

While the list of Kepler planetary candidates contains majority of the knowledge we have about exoplanets, Fressin said the catalog is not yet complete, and the catalog is not pure. “There are false positives from events such as eclipsing binaries and other astrophysical configurations that can mimic planet signals,” Fressin said.

By doing a simulation of the Kepler survey and focusing on the false positives, they can only account for 9.5% of the huge number of Kepler candidates. The rest are bona-fide planets.

This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft. Credit: C. Pulliam & D. Aguilar (CfA)

This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft. Credit: C. Pulliam & D. Aguilar (CfA)


Altogether, the researchers found that 50 percent of stars have a planet of Earth-size or larger in a close orbit. By adding larger planets, which have been detected in wider orbits up to the orbital distance of the Earth, this number reaches 70 percent.

Extrapolating from Kepler’s currently ongoing observations and results from other detection techniques, it looks like practically all Sun-like stars have planets.

The team then grouped planets into five different sizes. They found that 17 percent of stars have a planet 0.8 – 1.25 times the size of Earth in an orbit of 85 days or less. About one-fourth of stars have a super-Earth (1.25 – 2 times the size of Earth) in an orbit of 150 days or less. (Larger planets can be detected at greater distances more easily.) The same fraction of stars has a mini-Neptune (2 – 4 times Earth) in orbits up to 250 days long.

Larger planets are much less common. Only about 3 percent of stars have a large Neptune (4 – 6 times Earth), and only 5 percent of stars have a gas giant (6 – 22 times Earth) in an orbit of 400 days or less.

The researchers also asked whether certain sizes of planets are more or less common around certain types of stars. They found that for every planet size except gas giants, the type of star doesn’t matter. Neptunes are found just as frequently around red dwarfs as they are around sun-like stars. The same is true for smaller worlds. This contradicts previous findings.

“Earths and super-Earths aren’t picky. We’re finding them in all kinds of neighborhoods,” says co-author Guillermo Torres of the CfA.

Planets closer to their stars are easier to find because they transit more frequently. As more data are gathered, planets in larger orbits will come to light. In particular, Kepler’s extended mission should allow it to spot Earth-sized planets at greater distances, including Earth-like orbits in the habitable zone.

Kepler detects planetary candidates using the transit method, watching for a planet to cross its star and create a mini-eclipse that dims the star slightly.

Sources: Harvard Smithsonian CfA, AAS Press Conference

About 

Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

Beck45 January 7, 2013 at 9:19 PM

It’s an amazing time to be alive with these early results coming in… Funny how stories like this get no mainstream news coverage but reality shows and professional sport are apparently of maximum importance.

elvigy January 7, 2013 at 11:58 PM

Bread and circuses. :(

jakedimare January 7, 2013 at 9:23 PM

“De revolutionibus orbium coelestium”

Martin January 7, 2013 at 9:29 PM

Will it become possible in the near future (50 years?) to detect liquid water and/or vegetation on planets outside the solar system?

Kevin Frushour January 8, 2013 at 1:16 AM

Considering that 20 years ago we had no evidence of even a single planet other than our nine? I’d say high.

Lorin Ionita January 8, 2013 at 10:44 AM

It should be easier when new and bigger telescopes come online in the next 10-20 years.

TheDirtBoy January 8, 2013 at 5:58 PM

Direct detection of liquid water or vegetation? Not likely within the next century.
Analysis of the atmosphere to detect water or oxygen or even synthetic compounds, we are practically there now. Within the very near future we will be able to infer surface conditions based on atmospheric content, but unfortunately what your looking for is still a long ways off.

Torbjörn Larsson January 8, 2013 at 8:06 PM

Water vapor has been detected, but so far in atmospheres hot uninhabitable planets near their stars.

The larger telescopes coming on line within the decade, including JWST, should be able to do this on nearby habitable zone terrestrials I think. Any chlorophyll analog “red edge” wouldn’t be dependent on the scope being space based AFAIK. Water (and oxygen, methane et cetera) is iffier, but they come up with tricks all the time.

Be a Mentor January 8, 2013 at 12:56 AM

Can you imagine the resources available on a super earth. I wouldn’t even know what to expect.

Tar Atanamir January 8, 2013 at 9:17 AM

Recourses? If we find a new earth and can go there, do you seriously think that we will use it as a source for resources? If we have the capability to go to another solar system then going to Mars would be a cakewalk, and the usable surface of mars is just as big as Earths. And also, greenhouse gas producing factories would be a good thing on Mars.

Peristroika January 8, 2013 at 3:35 PM

“do you seriously think that we will use it as a source for resources?” And WHY wouldn’t we?? If you get to a new Earth, aren’t you going to mine for metal? (No, we can get it sent from old, used up Earth.” You are going to try to get rich just like you do here. You are going to live and not pretend you are a self-sustaining ecosystem unto yourself.

Tar Atanamir January 8, 2013 at 6:00 PM

By the time we have the tech-level to colonize a planet in another solar system I would assume that the post-scarcity society will have been in place for quite a while. And probably fueled by turning Mars into a factory planet.

Peristroika January 8, 2013 at 8:41 PM

Be that as it may, Mars will be light years away. When I need a widget, I’m not sending sub-space word to ye olde solar system. Most systems will have their own Mars’s. You’re talking science fiction where we can break the speed of light barrier. No, any system we go to will need to be self sufficient. The distance is the over riding factor. Besides, can you imagine the energy required to send millions of tons of steel and aluminum and iron at near light speeds across the cosmos? T’aint gonna happen.

Kevin Frushour January 8, 2013 at 1:17 AM

Has to be life out there. With 17 billion different planetary developments in this galaxy alone – it’s too high to ignore anymore.

Aqua4U January 8, 2013 at 2:05 AM

Changes the Drake equation? N = R* • fp • ne • fl • fi • fc • L

N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
R* =The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life bearing planets on which intelligent life emerges.
fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.

And therefore increases the probability of ETC’s!

Stan Taylor January 8, 2013 at 1:06 PM

Solutions to the Drake equation change dramatically when the variables are given new values – i.e. change “we don’t know” to “approximately a zillion.”

M_Malenfant January 8, 2013 at 10:41 AM

These numbers seem to be still ‘worst case’ values considering only what can be directly measured with Kepler (and other current methods). I’d be interested in a serious extrapolation to higher distances respectively lower masses at higher distances. I’m pretty sure we’ll get a significantly higher number of planets.

Until now the spectrum of planetary system structures has become visible only with it’s relatively extreme fringes. The number of planetary systems found at least to me suggests, that not only individual planets, but also sizable planet systems are rather the norm than an exception.

Torbjörn Larsson January 8, 2013 at 8:00 PM

I was going to post something on that anyway, seeing an earlier albeit flawed naive estimate. So I did, see my longer comment.

How planetary systems develop will have a major effect on this. Perhaps planets are usually packed around their stars.

Or perhaps we live in an unusually planet and habitable poor sparse system. In any case, seeing how most stars are M stars and will have many habitables either way, I see it as another nail in the coffin of Rare Earth ideas.

Jon Souter January 8, 2013 at 11:32 AM

Kepler is an amazing mission, it has truly really revolutionised planet hunting.

When you consider how short a time Kepler’s been observing (~4 years) in combination with how long it takes a far-flung planet such as our own Neptune to orbit the sun (>160 years!) – the numbers we’re seeing so far are still very conservative.

We’d have to observe each star for several human lifetimes to get a more complete picture of the quantity of planets out there.

There sure is no shortage of planets out there, way beyond our reach !

GregtheThird January 8, 2013 at 2:20 PM

I was looking at the Kepler data in order to answer a specific question, but it seems that they have not included the metallicity data for the stars they are studying. On their table they do include the metallicity of the star if the finding is a false positive but not for their candidate planets. Was this a deliberate omission?

Torbjörn Larsson January 8, 2013 at 7:55 PM

Corot, and perhaps now Kepler, data implies that metallicity in the later star generations has no effect on the frequency of terrestrials and Neptunes. (Eg the first stars had no planets.) Low metallicity only means slightly fewer giants AFAIK, see my lengthy comment why.

So maybe they just didn’t find it a vital characteristic for their statistics.

Torbjörn Larsson January 8, 2013 at 7:52 PM

This will scrap the old 1/r^2 model, where the smallest planets were the most frequent. But from the metallicity dependence of gas giants (Jupiters) vs the non-independence of the rest it is apparent that the different formation processes shows up.*

On the other hand, there is now planets galore!

The minimum estimate from Kepler is now ~ 100 – 200 billion planets, and considering that the Milky Way has something like 200 billion stars (downadjusted in the latest estimates), we have perhaps ~ 1 bound planet/star. Microlensing estimates is ~ 1 unbound planet/star.

But Kepler’s current data pipeline, according to released statistics, has ~ 18 000 candidates around ~ 11 000 stars! [ http://arxiv.org/pdf/1212.2915v1.pdf ] The pipeline is ~ 98 % accurate, and as we see here the earlier estimate of ~ 90 % true positives is verified.

If we don’t consider planetary system formation models that seem to pack planet closer to the star, as our own system’s formation model (the Nice model) tests well, we can do naive average estimates.

Using our own system with 8 planets, we get ~ 9*200*10^9 ~ 2*10^12 Milky Way planets from bound and unbound planets.

However, if Kepler would be looking at our system it would see Earth by now, no more than one planet because our larger system is a bit wobbly and few planets line up, and if so by ~ 0.3 % probability. Hence a Kepler looking at 100 000 Sun like stars should see ~ 300 planets.

But it sees nearly 2 orders of magnitude as many planets. If outer planets continues the trend we see that the Milky Way could have on average ~ 2*10^13 planets! We can also estimate naively that our system is unusually sparse and planet poor.

Since the observable universe has ~ 200 billion large galaxies in the newest estimates, and the universe is at least 10 times as large in radius or it wouldn’t be as uniform, the universe could have ~ 10^13*10^11*10^3 ~ 10^27 or more planets.

As for habitables, the Habitable Exoplanet Catalog has right now ~ 40 % of the habitables from systems with at least 2 habitables. And there has been a recent observation of a system with 3 habitables, albeit with controversial stability. [ http://io9.com/5969407/astronomer-discovers-three-potentially-habitable-planets-orbiting-around-one-red-dwarf ]

A naive estimate would add that our system is also poor on habitables, not unusually so but not a premier estate either.

* The model agrees with that gas giants form by core collapse. Tested by that low metallicity stars scatters their disk earlier, and gas giants therefore doesn’t grow as large.

The Sun is a relatively high metallicity, just at the end of the top fo the distribution, and so the presence of Jupiter and Saturn is consistent.

hillsider62 January 9, 2013 at 5:02 AM

Wow. I love science fictions.

Tar Atanamir January 9, 2013 at 10:10 AM

What I am saying is that traveling to another solar system is so extreme that it will not happen until FAR into the future!
And when we do get the capability to do so, we will already have a post-scarcity economy…

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