Recent research makes it seem like astronomers can’t look up without finding exoplanets. Data illustrate scores of super earths, planets in their habitable zones, and multiple-planet systems… But a specific type of planet has proven elusive: a planet orbiting at a considerable distance from its parent star.

Gemini Observatory’s Planet-Finding Campaign recently completed the most extensive direct imaging survey to date, but the results were mostly devoid of large planets—especially gas giants—at significant distances from their parent stars. This may seem counter-intuitive… After all, we think of our own solar system as ordinary or average, and it includes giant planets such as Uranus and Neptune, which orbit quite far away from our sun.

Michael Liu, leader of the Gemini Planet-Finding Campaign, sums up the situation this way: “We’ve known for nearly 20 years that gas-giant planets exist around other stars, at least orbiting close-in. Thanks to leaps in direct imaging methods, we can now learn how far away planets can typically reside. The answer is that they usually avoid significant areas of real estate around their host stars. The early findings, like HR 8799, probably skewed our perceptions.”

Exoplanet discoveries are usually based on data taken from the parent star, but HR 8799 was one of the first star systems observed directly from Earth. Using the Gemini telescope, researchers could see gas-giants at large orbital distances from their sun. At the time of discovery in 2008, they did not have enough background knowledge to realize that HR 8799 was very, very unusual.

But gas giants aren’t missing; they just tend to cling to their parent stars in a close orbit. And this lack of distant gas giant planets is apparent across all sizes and types of stars.

Difficulty finding planets at distant orbits has a silver lining, because absent planets can actually tell us more about planet formation. Astronomers are developing an explanation for the strange holes in dust disks surrounding young stars. “It makes sense that where you see debris cleared away that a planet would be responsible, but we did not know what types of planets might be causing this. It appears that instead of massive planets, smaller planets that we can’t detect directly could be responsible,” said Zahed Wahhaj of the European Southern Observatory.

Even though the missing planets have taught us something, the search for planets with orbits similar to that of Uranus and Neptune continues. And we thought we lived in an average solar system…

Alyssa Keimach is an astronomy and astrophysics student at the University of Michigan and interns for the Morrison Planetarium.

Image credit: NASA/ESA/C.Carreau

… Or went, anyway, until last week. An international team of scientists led by Alessandro Morbidelli, presenting at a planetary sciences meeting in France, knocked the large impact theory on its side.

There has always been a significant flaw in the notion of a body a few times more massive than Earth colliding with Uranus. The bright blue planet’s 27 known moons should have continued to orbit the planet at their original angles, but they too lie at almost exactly 98 degrees.

Using computer simulations, Morbidelli and his team examined the large impact theory. The researchers soon discovered that the collision must have happened early in Uranus’ history. The scientists found that if Uranus had been hit when still surrounded by a protoplanetary disk—the material from which the moons would form—then the disk would have reformed into a fat doughnut shape around the new, highly-tilted equatorial plane. Collisions within the disk would have flattened the doughnut, which would then go on to form the moons in their current positions.

From National Geographic News:

When Uranus was hit, this disk was disrupted but then reformed around the planet’s tilted equator, eventually giving rise to the moons in the positions we see today.

However, the simulation also threw up an unexpected result: in the above scenario, the moons displayed retrograde motion—meaning they orbited in the opposite direction to that which we observe. Morbidelli’s group tweaked their parameters in order to explain this.

The surprising discovery was that Uranus was not tilted in one go, as is commonly thought, but rather that two or more smaller collisions transformed the system, tilting the planet and giving the moons the orbits we observe today.

This research is at odds with current theories of how all planets form—not just Uranus, says Morbidelli.

The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk. They should have suffered no giant collisions. The fact that Uranus was hit at least twice suggests that significant impacts were typical in the formation of giant planets. So, the standard theory has to be revised.

Image: Lawrence Sromovsky, University of Wisconsin-Madison, Keck Observatory