This December, Jupiter will rival the brilliance of Venus in the night sky as Earth makes its closest approach to the giant planet for this year (no doubt 2012 soothsayers will mistake it for the final approach of the mythological doomsday planet Nibiru).
December's "Christmas Star" appearance of Jupiter will belie the fact that this monster, 318 times the mass of Earth, had the potential to snuff-out Earth's formation 4.5 billion years ago.
That's the conclusion from a new analysis of the NASA Kepler space telescope survey of planets orbiting other stars. A team of astronomers searched the Kepler data for any Earth-sized planets cohabiting in systems with "hot Jupiter" class planets.
The so-called "hot Jupiters" are so close to their stars they complete and orbit within a matter of days (Jupiter takes 12 years). The only way they could have gotten that close to their star was to somehow migrate from a much farther out frigid planetary "construction shipyard" where ices and lightweight gasses could condense onto a solid body.
Kepler can't photograph planets, but it measures the very slight dimming of a star when a planet in an edge-on orbit passes in front of the star. We'll be treated to such an event this Jun. 5-6, when Venus makes a rare transit across the face of our sun.
Looking at 63 candidate hot Jupiter systems in the Kepler data, the research team did not find any evidence for small companion terrestrial planets. The hot Jupiters complete an orbit so quickly that astronomers can look for irregularities in the timing of the transits — known as transit timing variations, or TTVs — that would be telltale evidence for the pull of smaller, unseen bodies.
But no TTVs were detected. The absence of small, Earth-sized worlds near hot-Jupiters suggests the migration of the hot Jupiter planet prevented or aborted the formation of smaller terrestrial planets.
The team, led by Jason Steffen of the Fermilab Center for particle Astrophysics, published their results in the May 7 edition of Proceedings of the National Academy of Sciences.
Odds of finding a smaller companion planet got a little better when researchers expanded the search to include systems with close-orbiting "hot Neptunes" of around 15 Earth masses. Of the 222 hot Neptunes in the Kepler database, there were two with possible companions that changed the transit timing of the giant planets' orbits.
The team also looked at so-called "warm Jupiters" that migrate not as close to the star as hot Jupiters. They had about a ten percent chance of companion planets.
These results suggest that the hot Neptunes and warm Jupiters had a markedly different formation history from the hot Jupiters.
In our solar system, there is a growing body of evidence that suggests there was a lot of planet migration in our early planetary history. The giant outer planets moved, well, outward.
This shoveled icy bodies outward to form the Kuiper belt, where Pluto dwells. What's more, Jupiter's gravitational influence formed the asteroid belt by keeping this primordial debris from agglomerating into an Earth-sized planet — as dramatically demonstrated last week by a detailed analysis of the mini-planet/asteroid Vesta from NASA's Dawn mission.
If Jupiter's orbit was more elliptical, the asteroid belt would extend all the way through the area of the inner solar system and Earth would be a bunch of debris, like the blown-up planet Alderaan in the Star Wars trilogy (though Earth would have not formed in the first place).
How could the early solar system have been different enough for Jupiter to migrate? If massive enough spiral gas density waves formed in the protoplanetary disk surrounding the newborn sun, it could have caused Jupiter to spiral inward like a railroad car riding the tracks.
Another possibility is that Jupiter could have played planetary pinball with two or more other less massive gas giant planets. The losers in this musical chairs match might have either been ejected from the solar system or plunged into the sun.
At first glance it seems there are many hot Jupiters out there. In 2006 The Hubble Space Telescope looked clear into the heart of our galaxy and found 16 hot Jupiters. Statistically, this means there are at least 7 billion hot Jupiter-class planets in our galaxy.
But it's estimated that our galaxy contains at least 100 billion planets. And so, in that context, hot Jupiters are comparatively rare. That's a good thing for assuring us that there are plenty of places for Earth-like planets to hang out.
Image credits: D. Agular/CfA, NASA