Hunt is on for “Killer” Third Star in BD+20 307 Binary System

by Ian O'Neill on January 10, 2009

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Exoplanet collision in BD+20 301. Possibly an Earth-like rocky exoplanet was involved? (Lynette Cook)

Exoplanet collision in BD+20 301. Possibly an Earth-like rocky exoplanet was involved? (Lynette Cook)

In September, it was announced the Chandra X-ray Observatory had spotted something very odd about BD+20 307. The binary system appeared to have a dusty disk surrounding it, indicative of a young, planet-forming system a fraction of the age of the Solar System. However, it was well known that the binary was actually several billion years old. It turns out that this disk was created by a rare planetary event; a cataclysmic planetary collision.

On Wednesday, at the AAS conference in Long Beach, I attended the “Extrasolar Planets” session to listen in on more results from Hubble about the exciting exoplanet discoveries in November… however, for me, the most captivating talk was about the strange, dusty old binary and the future detective work to be carried out to track down a planet killer…

The talks by astrophysicists working with the optical Hubble data were superb, showing off some of the science behind last years spate of direct observations of exoplanets, particularly the massive planet orbiting the star Fomalhaut, shaping a scattered disk of dust. However, there was no further news to report, apart from some cool numerical models the scientists will be using to characterize Fomalhaut b and a very interesting talk about the predicted lifetimes of exoplanets undergoing tidal stresses (which, unfortunately, I missed the first five minutes of as I got lost in the Long Beach Convention Center).

The one presentation that did pique my interest was Ben Zuckerman’s review of the progress being made in the study of BD+20 307. A few months ago, this piece of research caused a huge amount of interest as it provided the first piece of evidence of a huge, rocky planetary collision in the star system 300 light years away. Naturally, many news sources ran with article titles like: Is this what the Solar System would look like after Earth was hit by another planetary body? As Zuckerman pointed out, the fact that the group used an artist impression of a colliding Earth-like body (plus land masses and oceans, as pictured top) was no accident. BD+20 307 is certainly at an age when oceans might have formed and life–as Zuckerman morbidly conjectured–may have thrived. Not for any longer

Usually when we observe dust around a star, we can assume that it is a planet-forming star system that is fairly young. Conversely, as I found out to great depth in the conference, very old white dwarf systems can reveal a lot about their past planetary population when their dusty contaminants are studied. However, the dust contained in the BD+20 307 system is a puzzle. Astronomers had discovered a system, of comparable age to ours with a large amount of warm dust (T~500K). A system of that age will have long since expelled (via stellar wind pressure) or accreted any left-over dust from the planet-forming stages. Therefore, the only remaining explanation is that a rocky body collided with another, ejecting a huge amount of micron-sized warm dust particles.

So is this what the Solar System would look like after Earth is shattered by another planet? Possibly.

Zuckerman then pushed into some work being done to understand how the planetary collision could have happened in the first place. After all, the planets in our Solar System have settled into long-term stable orbits, any planet in BD+20 307 will have the same qualities. There were some questions as to whether the binary stars may have contributed to destabilizing the system, but Zuckerman quickly argued against this idea as the binary has such a tight orbit (with an orbital period of only 3.5 days), the destroyed planet will have found a stable orbit far from any gravitational variations.

So what could have caused the carnage in BD+20 307? We know that massive planetary bodies exert a huge gravitational pull on their host star and other planets in a system (i.e. Jupiter in the case of our Solar System), occasionally bullying (and sometimes capturing) them along the way. A small nudge in the wrong direction and planets could be knocked from their orbits, set on a collision course. So, much effort is now being put into a search for a third, faint star in BD+20 307. Perhaps it could be orbiting far away from the dancing binary, occasionally swinging past the planetary bodies, setting up the huge collision event.

This certainly seems reasonable, as 70% of binary star systems are found to have a third star. However, Zuckerman’s team have yet to find the “killer” third star and he appears confident that after careful analysis that there is no other stellar body within a 20 AU radius of the binary pair. Next, he intends to study the “wobble” of the centre of mass of the BD+20 307 binary to see if there is any gravitational anomaly as the mysterious “third star” tugs at the pair.

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Hello! My name is Ian O'Neill and I've been writing for the Universe Today since December 2007. I am a solar physics doctor, but my space interests are wide-ranging. Since becoming a science writer I have been drawn to the more extreme astrophysics concepts (like black hole dynamics), high energy physics (getting excited about the LHC!) and general space colonization efforts. I am also heavily involved with the Mars Homestead project (run by the Mars Foundation), an international organization to advance our settlement concepts on Mars. I also run my own space physics blog: Astroengine.com, be sure to check it out!

Jorge January 13, 2009 at 4:14 PM

No, that’s not true.

There’s no “tug of war”. There are orbital preturbations on Earth caused by Jupiter and every other body in the Solar System. These existed since the system formed, and nowdays Jupiter’s are the largest because it is by far the most massive of the planets and is relatively close to us. Back in the days before the impact that gave birth to the Moon, the impacting planet might have gotten close enough to perturb more the Earth’s orbit than Jupiter, but since then, the big perturber has always been Jupiter.

These perturbations don’t “pull the Earth out of its orbit” either. In fact, the notion that the Earth follows a perfect ellipse around the Sun is wrong. The Earth (and any other planet) follows a path that is always changing and that after a large number of revolutions defines a region of space with the shape of a donut.

The Earth gets pulled in by Jupiter every time it passes it, which happens more or less every 13 months. When that happens near the aphelion, every 12 years or so (Jupiter’s orbital period) the pull is slightly larger, and therefore the perturbation is also slightly larger. But the keyword here is “slightly”. Nothing that could be described by “tug-of-war”; not by a long shot.

Tim January 22, 2009 at 10:55 AM

I think what he’s proposing isn’t that outlandish. We’re not talking about a star that orbits just one other star and a few small planets. This planet is larger than Jupiter and the stars are more than likely larger than our sun which means that the outer star’s orbit is not as well defined as any planets in our own solar system.

So if the star ends up taking a swing closer than usual to the other two stars or closer than usual to the planet, it will tend to have a much larger effect upon that planet’s orbit warping it enough to cause an impact that otherwise might not have taken place. This could very well cause interplantary collisions and asteroidal confrontations that would normally not happen in a single-star solar system like our own.

If that system was like our own, you would be right. The degree of magnitude of pull on the orbit of any celestial being would be much, much less. I don’t think they’re comparable in the way you’re suggesting based on the combined gravitational pull of three stars.

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