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

Weather on Uranus

Twenty-six years ago, when Voyager 2 took this image of Uranus, scientists were disappointed by the seemingly smooth surface of the distant planet. But earlier this week, astronomers displayed a wholly different picture (see right) of Uranus—sultry and stormy!

It took the Earth-based Keck Observatory to capture what Voyager 2 couldn’t, despite being only 50,000 miles from Uranus in 1986. Keck took sharp, high-resolution infrared images showing bizarre weather on the sideways planet.

The planet’s deep blue-green atmosphere is thick with hydrogen, helium and methane. Winds blow mainly east to west at speeds up to 560 miles per hour, in spite of the small amounts of energy available to drive them.

Scientists believe that the primary driving mechanism for these storms must be solar energy because there is no detectable internal energy source. “But the Sun is 900 times weaker there than on Earth because it is 30 times further from the Sun, so you don’t have the same intensity of solar energy driving the system,” says planetary scientist Larry Sromovsky. That might explain why storms on Uranus are much less violent than those on Earth.

Sromovsky and colleagues announced their findings this week at a meeting of the American Astronomical Society’s Division of Planetary Sciences.

What were you thinking?

That’s what we would like to ask local Russ George. This week, several news outlets report how George dumped 100 tons of iron into the Pacific Ocean off the British Columbia coast illegally.

George has a history of geoengineering attempts like this one. This time a native Canadian group hired him to reduce carbon in the ocean to boost dwindling salmon populations. If you recall from a story we ran last summer about this technique, iron can feed algal blooms, which then sink to the ocean floor, sequestering carbon as they do.

But tampering with the ocean like this is obviously dangerous and highly regulated. According to the New York Times, George dumped ten times as much iron as the experiment mentioned in our story and violated two international agreements on geoengineering.

Tigers, Detroit and otherwise

Even before Detroit finished New York off in the American League Championship Series on Thursday, talks of trading Yankee star Alex Rodriguez were rampant.

His post-season performance was so bad, he was benched for the last two games. And we all know how much the Yanks like to win. But A-Rod’s contract looks to be a losing proposition for the team—even if they are able to unload him, they’ll likely have to pick-up part of his remaining salary. Which is huge, according to ESPN,

Rodriguez, who will turn 38 next July, is signed for the next five years and guaranteed another $114 million.

In addition, his contract includes a marketing agreement with the Yankees that could add as much as $30 million to the deal…

Such a waste. If you’ll remember, a couple of years ago we discussed how his large salary could help save wild tigers. Who could use his help right about now. The Guardian reports that an Indian court lifted a ban on tourism in tiger reserves this week. Officials are hoping tourists and the tiger habitats can co-exist. Hopefully they have more luck co-existing than A-Rod and the Yankees.

Image: Lawrence Sromovsky, Pat Fry, Heidi Hammel, Imke de Pater

Previous studies have noted that a “dynamical instability” (which is to say, a complex interaction of gravitational effects of different planets on one another) affected the orbits of giant planets when the solar system was a mere 600 million years old. As a result, the giant planets and smaller bodies scattered away from each other… A little bit of self-segregation.

Some small bodies migrated into the Kuiper Belt and others traveled farther inward, producing impacts on the terrestrial planets and the Moon. The giant planets shifted around as well. Jupiter, for example, scattered most small bodies outward and moved inward.

Jupiter played one of the biggest roles in the solar system’s youth. Scientists believe it protected smaller planets, like our own, from colliding with each other. Scientists explain the giant world’s protective status through the “jumping Jupiter” theory. “They proposed that Jupiter’s orbit quickly changed when Jupiter scattered off of Uranus or Neptune during the dynamical instability in the outer solar system,” says David Nesvorny of the Southwest Research Institute.

But when Nesvorny ran computer simulations of this “jumping Jupiter” theory, he ran into a problem. While Jupiter did in fact jump through interactions with Uranus or Neptune, the simulations also showed that Uranus or Neptune got knocked out of the solar system.

So Nesvorny wondered whether the early solar system could have had five giant planets instead of four. By running the simulations with an additional giant planet with mass similar to that of Uranus or Neptune, things suddenly fell in place.

Nesvorny believes that jumping Jupiter ejected one planet from the solar system, leaving the four gas giant planets we know and love behind. Thankfully, Jupiter jumped, leaving the terrestrial planets (including Earth) undisturbed.

“The possibility that the solar system had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence,” says Nesvorny.

This research appears in a recent edition of the Astrophysical Journal Letters.

Image: Southwest Research Institute

… 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