How Common are Solar Systems Like Ours?

by Nancy Atkinson on January 5, 2010

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Solar system montage. Credit: NASA

On the whole, we’d like to think we’re special, but we also hope we aren’t alone in the Universe. Astronomers have been trying to figure out just how common solar systems like ours are across the cosmos, and during one moment of epiphany one scientist figured out how to make the calculations. It took a worldwide collaboration of astronomers to do the work, but they concluded that about 10 – 15 percent of stars in the universe host systems of planets like our own, with several gas giant planets in the outer part of the solar system.

“Now we know our place in the universe,” said Ohio State University astronomer Scott Gaudi. “Solar systems like our own are not rare, but we’re not in the majority, either.”

The find comes from a collaboration headquartered at Ohio State called the Microlensing Follow-Up Network (MicroFUN), which searches the sky for extrasolar planets.

MicroFUN astronomers use gravitational microlensing — which occurs when one star happens to cross in front of another as seen from Earth. The nearer star magnifies the light from the more distant star like a lens. If planets are orbiting the lens star, they boost the magnification briefly as they pass by.

During his talk at the American Astronomical Society meeting in Washington, DC today, Gaudi said, “Planetary microlensing basically is looking for planets you can’t see around stars you can’t see.”

This method is especially good at detecting giant planets in the outer reaches of solar systems — planets analogous to our own Jupiter.

This latest MicroFUN result is the culmination of 10 years’ work — and one sudden epiphany, explained Gaudi and Andrew Gould, professor of astronomy at Ohio State.

Ten years ago, Gaudi wrote his doctoral thesis on a method for calculating the likelihood that extrasolar planets exist. At the time, he concluded that less than 45 percent of stars could harbor a configuration similar to our own solar system.

Then, in December of 2009, Gould was examining a newly discovered planet with Cheongho Han of the Institute for Astrophysics at Chungbuk National University in Korea. The two were reviewing the range of properties among extrasolar planets discovered so far, when Gould saw a pattern.

“Basically, I realized that the answer was in Scott’s thesis from 10 years ago,” Gould said. “Using the last four years of MicroFUN data, we could add a few robust assumptions to his calculations, and we could now say how common planet systems are in the universe.”

The find boils down to a statistical analysis: in the last four years, the MicroFUN survey has discovered only one solar system like our own — a system with two gas giants resembling Jupiter and Saturn, which astronomers discovered in 2006 and reported in the journal Science in 2008.

“We’ve only found this one system, and we should have found about eight by now — if every star had a solar system like Earth’s,” Gaudi said.

The slow rate of discovery makes sense if only a small number of systems — around 10 percent — are like ours, they determined.

“While it is true that this initial determination is based on just one solar system and our final number could change a lot, this study shows that we can begin to make this measurement with the experiments we are doing today,” Gaudi added.

As to the possibility of life as we know it existing elsewhere in the universe, scientists will now be able to make a rough guess based on how many solar systems are like our own.

Our solar system may be a minority, but Gould said that the outcome of the study is actually positive.

“With billions of stars out there, even narrowing the odds to 10 percent leaves a few hundred million systems that might be like ours,” he said.

At the AAS conference today, Gaudi was awarded the Helen B. Warner Prize for Astronomy.

Source: AAS, EurekAlert

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

star-grazer west coast January 5, 2010 at 2:22 PM

IMHO, there are very many Solar Systems like ours, however, for advanced lifeforms, very rare throughout the Universes’ history!! The type of galaxy and location/environment within a galaxy tells me the Universe is quite a dangerous place for lifeforms to develope beyond a primitive/certain stage before getting wiped out again and again!!!

petrk January 5, 2010 at 2:50 PM

10 or 15 percent?? eurekalert.org write 15 percent
http://researchnews.osu.edu/archive/planetrare.htm

I thank you for your advice.

Torbjorn Larsson OM January 5, 2010 at 3:06 PM

That is great work! Of course, the frequency of habitable system will likely be larger, seeing that there are other possible habitats than Earth analogs. (Like, say, Europa.)

@ sgw:

Funny how already known Earth history tests the exact opposite hypothesis to your tale. It shows us that once life gets going it is really robust and not much happens outside of many survivable mass extinctions for various reasons.

There _is_ a galactic habitable zone to lay over the local frequency of habitats, namely a system age and perhaps a star metallicity requirement. Life isn’t built of hydrogen alone, nor are evolution of multicellular life frequent so takes time to develop.

flogger11 January 5, 2010 at 3:56 PM

So what % of these solar systems contain primitive life? What % contain advanced? Guessing at least 20% contain some kind life, of that maybe.0001% at most may contain advanced, “self aware”, beings. Just thinking….

William928 January 5, 2010 at 4:14 PM

A fascinating subject, my favorite aspect of astronomy. The article states that in the last 4 years MicroFUN has discovered only one solar system like our own, with two gas giants similar to Jupiter and Saturn. How does this make the system “like our own”? Are they assuming that because two gas giants exist that it follows that there is an earth-sized rocky planet in the system? AFAIU we’ve discovered other exoplanet systems containing multiple gas giants….

William928 January 5, 2010 at 4:41 PM

After reading the September 2006 UT article “Earth-sized planets are probably common”, I understand how the assumption is made that earth-sized rocky planets probably exist in gas giant systems. Looking forward to the first discovery, it’s only a matter of time! A very exciting era of astronomy!

Spoodle58 January 6, 2010 at 2:31 AM

We do not have enough evidence to suggest any numbers at present as to how common a system like ours is, even the Kepler spacecraft can’t provide that answer.

Torbjorn Larsson OM January 6, 2010 at 4:15 AM

@ William928:

How does this make the system “like our own”?

AFAIU it is a less common result to have several giants ending up outside the terrestrials. See the Nice model and follow up research that nowadays pretty much replicates our planet system history, AFAIU.

Much of what happened to Earth specifically ending up habitable may have to do with the Nice model scenario. The Last Heavy Bombardment resulting from Jupiter’s dynamics has supplied both volatiles that established the initial CO2/water environment that life started in (and then immediately at that), and possibly other factors that established our plate tectonics as well. IIRC it was a recent result of last year that mantle volatiles measurably _must_ mostly trace back to comet analogs (explaining isotope ratios), not the planetoid accretion before that.

So it is a relief to see that this type of scenario isn’t as rare as one could assume a priori.

@ flogger11:

maybe.0001% at most may contain advanced, “self aware”, beings.

That is an iffy question. Most current biologists seems to adhere to the contingency scenario of evolution. If we “played back” life from its initial state the evolutionary pathway would differ in a presumably chaotic way. (I.e. exponential divergence. Perhaps worse in this case, as permutations goes as factorials.) Or at least be unlikely to repeat itself.

OTOH one can use frequency of traits to establish likelihoods. For example, many metabolic pathways have been established, so that is a likely occurrence. (At least after the first one and with current cellular machinery.) “Well, duh!”

On the surface it seems that multicellularity, the main difference between having relatively few traits individually and cooperatively as in bacteria and having relatively many as in multicellular plants and animals, is easy. Bacteria cooperates in many ways and even form tight units of biofilms when necessary. And multicellularity has been acquired many times independently in eukaryotes.

But looking closer, it seems or at least is claimed that the rigidity of the protective cellular wall of bacteria prohibits true multicellulars. While the flexibility of the eukaryote cellular membrane changes things. Why that is, is unclear to me, as some plants also form rigid walls. ‘Tis a mystery for this non-biologist.

As eukaryotes only evolved once, and late at that, ~ 1 Ga after an initial ~ 3 Gy of bacteria, it is claimed to be one of those rare events. OTOH it could be that in the same way that preexisting life prohibits abiogenesis by efficient competition (and having changed biosphere chemistry), preexisting eukaryotes prohibit similes by having taken available niches. Again, I dunno.

Aodhhan January 6, 2010 at 5:53 AM

Gaudi’s study predicts that 15% of solar systems are generally made up like ours. In having a few gas giants on the outer part and rocky planets relatively near to the sun, with one being in the habitable zone.

Knowing this percentage doesn’t give us any measure about life existing on a planet. However, if accepted, this figure can be used in a formula to predict both primitive and advanced life forms.
For that, many of you brought up some good points. The planet in the habitable zone would have to have water, a magnetic field, and an atmosphere to support life. This is just the start of an equation to predict life.

Star-gazer brings up a point which also must be considered… There are many times in the past, where our species could have gone extinct. Even now that we have so much technology…. we could still easily become extinct. Perhaps by a pandemic or the release of nuclear weapons.

This isn’t really my realm of study, but it will be interesting to see if this figure is accepted, and someone uses it to put together a formula to predict other life; even better when we can prove it!

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