96P/Machholz 1は太陽系外起源の彗星かもしれない

米国アリゾナ州Lowell ObservatoryのDavid Schleicherによれば、96P/Machholz 1は太陽系外起源かもしれない。
[Stephen Battersby: "Has an alien comet infiltrated the solar system?" (2008/12/02) on NewScientist]

A comet orbiting our Sun may be an interloper from another star system.


Comet Machholz 1 isn't like other comets. David Schleicher of the Lowell Observatory in Flagstaff, Arizona, measured the chemical makeup of 150 comets, and found that they all had similar levels of the chemical cyanogen (CN) except for Machholz 1, which has less than 1.5% of the normal level. Along with some other comets, it is also low on the molecules carbon2 and carbon3.

Machholz 1彗星は他の彗星とは違っている。アリゾナ州FlagstaffにあるLowell ObservatoryのDavid Schleicherは150の彗星の化学組成を計測し、Machholz 1彗星以外は、同程度のシアン(CN)が存在することを発見した。Machholz 1彗星のシアンは他の彗星に比べて1.5%少なかった。他の彗星と比べて、Machholz 1彗星はC2炭素およびC3炭素3とも少なかった。

Schleicher suggests three possible explanations.

David Schleicherは次の3つの可能性を提案する。

The simplest is that Machholz 1 could have formed in an extremely cold region of the solar system. The other comets depleted in carbon2 and carbon3 are thought to have formed in the chilly outer regions of the Kuiper belt far beyond Neptune, where the low temperatures mean that most carbon gets trapped in other molecules. "In really extreme cold, maybe the cyanogen goes away as well?" says Schleicher.

最も単純な可能性は、Machholz 1彗星が太陽系の中の非常に冷たい領域で形成されたというもの。C2炭素およびC3炭素の少ない他の彗星は海王星よりもはるか彼方のカイパーベルトの極低温の領域で形成されたと考えられる。極低温では、炭素は他の分子に捕捉される。「実際の極低温では、シアンも同様に他の分子の取り込まれるかもしれない」とDavid Schleicherは言う。

Hurled out

A second possibility is suggested by the comet's peculiar orbit. Machholz 1 approaches very close to the Sun on its orbit, closer even than Mercury, so it is possible that repeated baking by the Sun's heat has removed most of its cyanogen.

第2の可能性は、彗星の独特な軌道である。Machholz 1彗星の軌道は水星よりも太陽近くに接近するので、太陽熱によってシアンが除去されてしまったかもしれない。

But a more exciting idea is that Machholz 1 is an alien. "An extrasolar origin makes it easy to explain the composition - of course we'd expect everything to be different," Schleicher told New Scientist. "Here, three molecules with carbon are all depleted, so maybe carbon is depleted across the board? That sounds to me as if it came from somewhere else and is not just an oddball from our solar system."

しかし、もっとエキサイティングの考えは、Machholz 1が太陽系外から来たというもの、「太陽系外起源は化学組成を簡単に説明できる。もちろん、すべてが違っていることを予測する。移動中に、炭素を持つ3つの分子が除去された? それはどこか他のところから来たと思わせる。そして、それは太陽系内の変わり者ではない。

Astronomers expect some comets to be hurled out of their stellar systems by the gravity of giant planets. In theory, they should be wandering around out there, and sometimes come flying though our system - although no comet has yet been found moving fast enough to be identified as an extrasolar interloper.


If Machholz 1 did fly in from interstellar space, a chance encounter with Jupiter could have made it a captive of our solar system.

Machholz 1彗星が太陽系外からやってきたとするなら、木星に接近して、太陽系に捕獲されたのかもしれない。

This would be an extraordinary discovery, but much more evidence is needed before the alien origin is any more than a hypothesis. Fortunately, Machholz 1 will be close to the Sun again in 2012, when astronomers can study its bright coma of gases again.

これは驚くべき発見だが、太陽系外起源が仮説以上のものとなるには、もっと多くの証拠が必要である。Machholz 1彗星は2012年に太陽に接近するので、天文学者たちは再びガスの明るいコマを観測できる。


Journal reference: Astronomical Journal (vol 136, p 2204)

David G. Schleicher 2008 The Astronomical Journal 136 2204-2213 doi: 10.1088/0004-6256/136/5/2204

David G. Schleicher
Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA

ABSTRACT. Narrowband-filter photometry of Comet 96P/Machholz 1 was obtained at Lowell Observatory during the comet's 2007 apparition. Production rates of OH, CN, C2, C3, and NH were derived from these data sets, and the quantity A(θ)fρ—a proxy measure of the dust production—was also calculated. Relative abundances, expressed as ratios of production rates with respect to OH (a measure of the water abundance), were compared to those measured in other comets. Comet Machholz 1 is shown to be depleted of CN by about a factor of 72 from average, while C2 and C3 are also low, but "only" by factors of 8 and 19, respectively, from "typical" composition (based on an update to the classifications by A'Hearn et al.). In contrast, NH is near the mid-to-upper end of its normal range. This extremely low CN-to-OH ratio for Machholz 1 indicates that it is either compositionally associated with Comet Yanaka (1988r; 1988 Y1), which was strongly depleted in CN and C2, but not NH2, or represents a new compositional class of comets, since Yanaka had a much greater depletion of C2 (>100×) than did Machholz 1 (8×). Evidence strongly suggests that the extremely anomalous compositions of these two comets are primordial rather than from recent thermal processing. It remains unclear whether these comets formed at a location in our solar system with unusual conditions and a low probability of being gravitationally perturbed into the inner solar system, or if one or both objects are interstellar interlopers.
なお、ここでDavid SchleicherがMachholtz 1とともに触れているYanaka彗星について、炭素が少ないことは1992年に知られている:
Uwe Fink: "Comet Yanaka (1988r): A New Class of Carbon-Poor Comet.", Science. 1992 Sep 25;257 (5078):1926-1929 17753496 (P,S,G,E,B,D)

As part of a program to determine the chemical composition of a sample population of comets, a very unusual comet, Yanaka (1988r), was observed in January 1989. Although the comet showed the usual emissions of Ol and NH(2), it did not display any hint of C(2) or CN emission. The comet is depleted in C(2) by at least a factor of 100 and in CN by a factor of 25 relative to typical comets. If comets originate from interstellar clouds, Yanaka (1988r) could be an interloper from a cloud of different composition. If Yanaka (1988r) was formed within our solar system, the solar nebula was less uniform than assumed by most present models of formation.

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