Is our solar system weird?

July 18, 2014 by Brian Koberlein, Universe Today
This artist’s view shows an extrasolar planet orbiting a star (the white spot in the right). Credit: IAU/M. Kornmesser/N. Risinger (

Is our Solar System normal? Or is it weird? How does the Solar System fit within the strange star systems we've discovered in the Milky Way so far?

With all the beautiful images that come down the pipe from Hubble, our Solar System has been left with celestial body image questions rivaling that of your average teenager. They're questions we're all familiar with. Is my posture crooked? Do I look pasty? Are my arms too long? Is it supposed to bulge out like this in the middle? Some of my larger asteroids are slightly asymmetrical. Can everyone tell? And of course the toughest question of all… Am I normal?

The idea that are suns with orbiting them dates back to early human history. This was generally accompanied by the idea that other would be much like our own. It's only in the last few decades that we've had real evidence of planets around other stars, known as exoplanets. The first extrasolar planet was discovered around a pulsar in 1992 and the first "hot jupiter" was discovered in 1995.

Most of the known exoplanets have been discovered by the amazing Kepler spacecraft. Kepler uses the transit method, observing stars over long periods of time to see if they dim as a planet passes in front of the star. Since then, astronomers have found more than 1700 exoplanets, and 460 stars are known to have multiple planets. Most of these stellar systems are around main sequence stars, just like the Sun. Leaving us with plenty of systems for comparison.

So, is our Solar System normal? Planets in a stellar system tend to have roughly circular orbits, just like our Solar system. They have a range of larger and smaller planets, just like ours. Most of the known systems are even around G-type stars. Just like ours.….and we are even starting to find Earth-size planets in the habitable zones of their stars. JUST LIKE OURS!

Not so fast…Other stellar systems don't seem to have the division of small rocky planets closer to the star and larger gas planets farther away. In fact, large Jupiter-type planets are generally found close to the star. This makes our rather unusual.

Computer simulations of early planetary formation shows that large planets tend to move inward toward their star as they form, due to its interaction with the material of the protoplanetary disk. This would imply that large planets are often close to the star, which is what we observe. Large planets in our own system are unusually distant from the Sun because of a gravitational dance between Jupiter and Saturn that happened when our Solar System was young.

Although our Solar System is slightly unusual, there are some planetary systems that are downright quirky. There are planetary systems where the orbits are tilted at radically different angles, like Kepler 56, and a sci-fi favorite, the planets that orbit two stars like Kepler 16 and 34. There is even a planet so close to its star that its year lasts only 18 hours, known 55 Cancri e.

Artist’s impression of the solar system showing the inner planets (Mercury to Mars), the outer planets (Jupiter to Neptune) and beyond. Credit: NASA

And so, the Kepler telescope has presented us with a wealth of exoplanets, that we can compare our beautiful Solar System to. Future telescopes such as Gaia, which was launched in 2013, TESS and PLATO slated for launch in 2017 and 2024 will likely discover even more. Perhaps even discovering the holy grail of exoplanets, a habitable planet with life…

And the who knows, maybe we'll find another planet… just like ours.

Explore further: Perth's planet hunter helps discover unusual exoplanet

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SnowballSolarSystem _SSS_
2 / 5 (8) Jul 18, 2014
Is our solar system weird?
No, the astro-programmers are weird.

I expect more from my NSF dollars than pebble accretion followed by planet migration. Has computer science now overtaken astrophysics too?

When I worked in industry I discovered that most young people don't want to go out onto the factory floor and get their hands dirty. All they want to do is sit behind a computer and push buttons all day, with this as the inevitable result. If you sufficiently fine tune starting conditions give yourself enough knobs to turn you can shoehorn even a solitary failed planet-formation model to fit any given stellar system.
1 / 5 (4) Jul 18, 2014
If you sufficiently fine tune starting conditions give yourself enough knobs to turn you can shoehorn even a solitary failed planet-formation model to fit any given stellar system.

This explanation doesn't only apply here, but in just about every aspect of science.

Perhaps their logic is flawed from the beginning. We know that a very high percentage of systems are binary.
What if our solar system formed as a binary system, with the sun being a companion star. The primary star would have been larger and burned out faster, and would explain why our gas giants formed farther away from the Sun. This would mean there's either a brown dwarf out there, or a black hole. There is evidence of some large body out in the Oort cloud affecting orbits of "objects":
SnowballSolarSystem _SSS_
1.6 / 5 (7) Jul 18, 2014
Perhaps their logic is flawed from the beginning. We know that a very high percentage of systems are binary.

Now you're cooking, with Jupiter and Saturn formed as hot Jupiters around a former binary Sun that spiraled in to merge at 4,567 Ma, forming the r-process radionuclides of the early solar system in stellar-merger nucleosynthesis. Then Uranus and Neptune would be hybrid-accretion (Thayne Currie 2005) super-Earths formed from circumbinary planetesimals 'condensed' by gravitational instability at the pressurized inside edge of our former circumbinary protoplanetary disk.
1 / 5 (4) Jul 18, 2014
Sounds plausible, dare I say even logical. Has there been any research you know of that has a hypothesis similar to this? I can't find any.
1 / 5 (2) Jul 18, 2014
Forget the core accretion theory of planet formation, which needs no end of starting condition tweaks to get anywhere near simulating our Solar System and can't explain the hundreds of "anomalous" exoplanetary systems we have so far discovered. And forget the disks of cosmic junk surrounding stars being "protoplanetary", because that begs the core accretion question.
Those systems are only anomalous to the core accretion theory. They are, however, perfectly normal for systems consisting of free planets that have been captured by stars while traveling past in all sorts of orientations.
5 / 5 (5) Jul 18, 2014
""Large planets in our own system are unusually distant from the Sun because of a gravitational dance between Jupiter and Saturn that happened when our Solar System was young."

"our Solar System is slightly unusual,"

In the Nice 2 model, more consistent with the new data on planetary formation and with the outher system moons than Nice, this dance is ubiqitous. It is the solar system outcome which is infrequent (at some 15 % IIRC). Which is lucky, seeing how most stars are M dwarfs and you want the tighter systems usually seen. [ http://en.wikiped..._2_model ]"
5 / 5 (6) Jul 18, 2014

"I expect more from my NSF dollars than pebble accretion followed by planet migration."

Science is untargeted except for getting results. And you have got them, that is what is observed.


"Forget the core accretion theory of planet formation".

Which no one can currently, since it is the main formation theory, and the only one that has been observed without theory constraints, twice. [ ]

"Those systems are only anomalous to the core accretion theory."

Wrong, see above! (And you can't square capture with the main observations of momenta and protoplanetary systems, the latter seen with protoplanets in formation no less.) Which is why you don't provide references.
not rated yet Jul 18, 2014
Umm, Kepler has found far more smaller planets nearest their sun. It appears large planets near the sun is actually more rare than our solar system make up.
SnowballSolarSystem _SSS_
1 / 5 (2) Jul 18, 2014

Indeed, 16 Cygni should be a good lab for metallicity studies, but even the most robust pebble accretion models couldn't tolerate a protoplanetary eccentricity of 0.689. It might, however, be a good candidate for disk instability, but if you insist on pebble accretion, then you're back to the problem of proposing a secondary mechanism for explaining away what doesn't fit.

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