Heavy Stellar Traffic Sends Dangerous Comets Our Way

New image of comet ISON
Comet C/2012 S1 (ISON) as imaged by TRAPPIST–South national telescope at ESO’s La Silla Observatory in 2013 (TRAPPIST/E. Jehin/ESO)

Sixty-six million years ago Earth underwent a cataclysmic change. Back then, our planet was dominated by dinosaurs, but a mass extinction event hastened the demise of these huge reptiles and paved the way for the mammalian takeover. Though there is some debate as to whether the extinction of the dinosaurs was triggered by an isolated disaster or a series of disasters, one event is clear — Earth was hit by a massive comet or asteroid and its impact had global ramifications.

The leading theory is that a massive comet slammed into our planet, creating the vast Chicxulub Crater buried under the Yucatán Peninsula in Mexico, enshrouding the atmosphere in fine debris, blotting out the sun for years.

Although there is strong evidence of comet impacts on Earth, these deep space vagabonds are notoriously hard to track, let alone predict when or how often they may appear. All we know is that they are out there, there are more than we thought, they are known to hit planets in the solar system and they can wreak damage of apocalyptic proportions.

Now, using fresh observations from the European Space Agency’s Gaia mission, astronomer Coryn Bailer-Jones, who works at the Max Planck Institute for Astronomy in Munich, Germany, has added an interesting component to our understanding of cometary behavior.

Stellar Traffic

Long-period comets are the most mysterious — and troubling — class of comet. They will often appear from nowhere, after falling from their distant gravitational perches, zoom through the inner solar system and disappear once more — often to be never seen again. Or they hit something on their way through. These icy bodies are the pristine left-overs of our solar system’s formation five billion years ago, hurled far beyond the orbits of the planets and into a region called the Oort Cloud.

In the Oort Cloud these ancient masses have remained in relative calm far from the gravitational instabilities close to the sun. But over the eons, countless close approaches by other stars in our galactic neighborhood have occurred, causing very slight gravitational nudges to the Oort Cloud. Astronomers believe that such stellar encounters are responsible for knocking comets from this region, sending them on a roller-coaster ride to the inner solar system.

The Gaia mission is a space telescope tasked with precisely mapping the distribution and motion of stars in our galaxy, so Bailer-Jones has investigated the rate of stellar encounters with our solar system. Using information in Gaia’s first data release (DR1), Bailer-Jones has published the first systematic estimate of stellar encounters — in other words, he’s estimated the flow of stellar traffic in the solar system’s neighborhood. And the traffic was found to be surprisingly heavy.

In his study, to be published in the journal Astronomy & Astrophysics, Bailer-Jones estimates that, on average, between 490 and 600 stars will come within 16.3 light-years (5 parsecs) of our sun and 19-24 of them will come within 3.26 light-years (1 parsec) every million years.

According to a press release, all of these stars will have some gravitational effect on the solar system’s Oort Cloud, though the closest encounters will have a greater influence.

This first Gaia data release is valid for five million years into the past and into the future, but astronomers hope the next data release (DR2) will be able to estimate stellar traffic up to 25 million years into the past and future. To begin studying the stellar traffic that may have been responsible for destabilizing the dinosaur-killing comet that hit Earth 66 million years ago will require a better understanding of the mass distribution of our galaxy (and how it influences the motion of stars) — a long-term goal of the Gaia project.

An Early Warning?

Spinning this idea into the future, could this project be used to act as an early warning system? Or could it be used to predict when and where a long-period comet may appear in the sky?

In short: “No,” Bailer-Jones told Astroengine via email. “Some close stellar encounters will for sure shake up the Oort cloud and fling comets into the inner solar system, but which comets on which orbits get flung in we cannot observe.”

He argues that the probability of comets being gravitationally nudged can be modeled statistically, but this would require a lot of assumptions to be made about the Oort Cloud, a region of space that we know very little about.

Also, the Oort Cloud is located well beyond the sun’s heliosphere and is thought to be between 50,000 and 200,000 AU (astronomical units, where 1 AU is the average distance between the sun and the Earth) away, so it would take a long time for comets to travel from this region, creating a long lag-time between stellar close approach and the comet making an appearance.

“Typically it takes a few million years for a comet to reach the inner solar system,” he added, also pointing out that other factors can complicate calculations, such as Jupiter’s enormous gravity that can deflect the passage of comets, or even fling them back out of the solar system again.

This is a fascinating study that goes to show that gravitational perturbations in the Oort Cloud are far from being rare events. A surprisingly strong flow of stellar traffic will constantly rattle otherwise inert comets, but how many are dislodged and sent on the long journey to the solar system’s core remains a matter for statistics and probability.


Battlestar Galactica’s “Twelve Colonies of Kobol” Star System Found?

An image at radio wavelengths of a young stellar quadruplet. Credit: CfA/Nature/Pineda
An image at radio wavelengths of a young stellar quadruplet. Credit: CfA/Nature/Pineda

825 light-years away, in the constellation of Perseus, hides one protostar and three previously unseen gas concentrations that are undergoing gravitational collapse — basically embryos of soon-to-be baby stars. Found through the analysis of data from radio telescopes by astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA), this tiny cluster of baby stars occupy a small volume only 10,000 AU across — meaning that they’d all easily fit within the confines of the boundaries of our solar system (yes, the Oort Cloud is the solar system’s outermost boundary).

This is exciting for a couple of reasons. Firstly, this little ‘stellar womb’ has given astronomers an opportunity to study the genesis of a multi-star system. Indeed, most stars in our galaxy belong to multi-star systems, whether that be binary or greater, and astronomers are currently trying to figure out whether they were born this way or whether, over time, stars jostled around and eventually became gravitationally bound. After analysis of the velocities of the protostar and stellar embryos, it appears that the masses are gravitationally interacting. In other words, it has the potential to mature into a quadruple star system in around 40,000 years, a minute amount of time in cosmic timescales. Although it is likely that the system will become unstable, possibly ejecting one or two of the stars in the process, it does provide observational evidence that multi-star systems can be born in a gravitational embrace.

A map of the Twelve Colonies via io9.com
A map of the Twelve Colonies via io9.com

But as I have a habit of linking astrophysical studies with science fiction imaginings, when I first saw this research, I immediately thought of the awesome re-imagined series’ Battlestar Galactica and Caprica.

Battlestar Galactica is set in the years following the Cylon attack on the Twelve Colonies of Kobol, which almost wiped out humanity in this far-flung part of the galaxy. The remaining survivors, headed by William Adama (Edward James Olmos), take to the stars in a fleet of ragtag spaceships in search of the fabled Earth. One of my favorite scifi storylines and favorite scfi TV shows. But I digress.

The Twelve Colonies consist of four stars — Helios Alpha, Helios Beta, Helios Delta and Helios Gamma — each with their own systems of planets, 12 in total, including capital world Caprica.

So that poses a question: Just because Battlestar Galactica imagines a quadruple star system (well, two binary systems in a mutual orbit), is it possible to have such a stable system of planets evolve in a multi-star system? Or are the gravitational interactions too complex for anything to coalesce and slot into stable orbits? Well, by understanding how multi-star systems evolve by finding examples like this embedded inside star forming molecular clouds, we may start to appreciate how common and how stable they are and whether accompanying planetary systems are a reality or something that will forever be confined to the Twelve Colonies.

READ MORE: Star Quadruplets Spied Growing Inside Stellar Womb (Discovery News)