What the Heck Is That and What Is It Doing There?

Gertrude Stein was supposed to have dissed the city of Oakland, CA, with the quip, "There is no there there," found in her book Everybody's Autobiography. She was actually describing how she couldn't find her childhood home on a visit to the city, her hometown, during a lecture tour through California.

Either way, astronomers examining the star HR 8799 have run into the opposite problem of a there where there shouldn't be one. There is very definitely a "there," or a fourth planet, orbiting it about 14.5 times as far away from it as the Earth is from the Sun. Astronomers call the distance from the Earth to the sun an "astronomical unit" or AU, meaning this fourth planet is 14.5 AUs from its star. There are also three other giant planets orbiting the star at much larger distances. The problem? None of the current theories that describe how large gas giant planets form allow for four such planets to be in a system of a relatively brand-new star like HR 8799.

Gas giants are different from small rocky worlds like our own Earth. All of their visible features are actually the top layers of their incredibly thick atmospheres, and exactly what kind of planet is at the center of those huge balls of gas isn't entirely clear. Whether they formed around some kind of gravitational instability that drew in material around it, or whether they formed when extremely large rocky cores gathered up far more gas than smaller worlds could isn't clear either. Both theories represent the best guesses and deductions of astronomers today.

Gravitational instability models only work for gas giants at least 30 AUs from their respective suns, and the core accretion models only work at 20 AUs or closer. So scientists have to scratch their heads at a single star that has four such planets, orbiting at the aforementioned 14.5 AUs, along with 25, 40 and 70 AUs respectively for the earlier trio. Planetary dynamics make it unlikely that planets of both kinds formed around the same star, and the current suspect for the weirdness is the very thick cloud of dust around HR 8799, which might have screwed up the quartet's orbits so much that they now hang out in very different neighborhoods than the ones in which they formed.

HR 8799 is about 129 light years from Earth, so we see it today as it actually was in 1821 or so. What if the real explanation for four gas giants being where they shouldn't is that an alien race has technology advanced enough to move planets around in their orbits? Well, waves from some of our first radio broadcasts will reach them sometime around the turn of the next decade, with higher and higher levels of activity building after that. So we'll know then, and we'll just hope that our own planet's current orbit matches their version of solar feng sui, lest they rearrange us or maybe otherwise alter things so as to improve their view of Venus.