May 30, 2020

AstroEvent: In Search of the Solar Birthplace.

Cancer & M67.

(Photo-graphic by Author; M67 image courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF).

A mystery lurks in the heart of the Crab. This month, as the Moon passes Full on March 6th and begins to once again leave the late evenings to us dark sky observers, I’d like to turn your attention to a non-descript open cluster in the constellation Cancer. Just a few degrees below the ecliptic plane and the Beehive Cluster (M44, also known as Praesepe) and about 1.8° degrees west of Alpha Cancri lies M67, a +6.1 magnitude open cluster located about 3,000 light years away. Composed of roughly 1,400 stellar members packed into a space about 10 light years across, this open cluster bucks the trend of early destruction and dispersal that is often the future fate of younger clusters such as the Pleiades and the Hyades and has an estimated age of 3.2 to 4.8 billion years. If that age range sounds familiar, that’s because our very own Sol and our solar system is estimated to be around 4.6 billion years old. This age relationship has placed M67 at the center of some controversy in recent years as a potential “birth-place” of, well, us. This cluster has escaped scattering largely because of its high inclination orbit around the galactic disk (note its position in the sky versus the Orion spur of our galaxy) and there is some conjecture that M67 is even the remnant of a small dwarf galaxy similar to one of the Magellanic Clouds.

But other factors make it attractive as our possible parent stellar nursery. In order for our solar system to have remained intact early in its history, the density of the cluster had to be such that a system-disrupting stellar encounter closer than 400 astronomical units (about ten times the Pluto-Sun distance) would have been improbable. Some evidence for such a far-out encounter early in our solar system’s history exists in the relatively placid orbits of the inner-planets versus the elliptical tilted orbits presented by modern day Trans-Neptunian & Kuiper Belt Objects.

M67 passes this bar, plus there’s something more. Abundances of beryllium, aluminum and iron isotopes found in asteroids suggest that in addition to spallation from cosmic ray bombardment, the solar system witnessed a massive supernova event within a third of a light year distant early in its history, “salting” the solar nebula with heavy elements. The abundance of heavy elements, known as the “metallicity” of our Sun (remember, astronomers think of every element except hydrogen and helium as “metallic”) is much higher than the ancient population stars found in ancient globular clusters. In fact, the exceptional abundance of such elements as carbon, nitrogen, and oxygen in our solar system is why we’re here to ponder this to begin with. Think of the metallic abundance in the barcode-looking spectra of the Sun as a stellar “finger-print” that just might link us to our kin…

Then in 2010, a study published by astronomers working at the University of Uppsala, Sweden published findings of a star in M67 named M67-1194 which has been found to be the most “Sol-like” star yet. Could it be a long lost cousin of our Sun? It has also been noted that from above the galactic plane, the orbits of Sol and M67 are nearly identical, both being about 27,000 light years out from the galactic core. M67 would have hosted several massive stars that would have been supernova candidates early in its history, and perhaps an early encounter with such as star would have flung us out of our home…

There are, however, problems with this scenario. First, although M67 and Sol have similar orbits around the Milky Way, it has been estimated that we’ve journeyed 27 times around the galactic center versus M67’s 17. Note that some of those parameters such as the cluster’s age aren’t precisely known, and only work if you accept the upper age limit. Also, M67 is currently 1,350 light years above the galactic plane versus our 20-90 light years; a recent study by Barbara Pichardo, Edmundo Moreno and Christine Allen published in the Astronomical Journal earlier this year demonstrates that the relative velocity between Sol and M67 is in the order of 20 kilometers per second; any said encounter sufficient to fling us out of the cluster would have also disrupted the nice, orderly inner solar system we see today and doomed Earth to a wondering ice-ball existence.

Add to that sobering scenario, sending our solar system from a cluster in a highly inclined orbit to a sedate one within the galactic habitable zone via a chance encounter is highly unlikely… or perhaps it was a 1 in a million cosmic shot that led to why we’re here as a sentient blogging species?

But whether M67 is our long lost home or not, it provides a fascinating study of where a solar system like ours could have came from; Astronomer Simon Zwart of the University of Amsterdam estimates that 10 to 60 of our Sun’s siblings may still lie with 320 light years of our solar system as we mutually revolve about the galaxy. A great many others will have been dispersed, and the odds are that our birth cluster no longer exists or lurks in a part of our galaxy obscured from view. The European Space Agencies’ Gaia mission to refine stellar positions and motions may help to pinpoint these stars, and is slated for launch in 2013. Whatever the case is, M67 makes a great binocular target spanning about 15’ arc minutes in northern hemisphere spring nights, and really comes of its own in a low power telescopic field littered with 10th magnitude stars. Its coordinates are;

Right Ascension: 08Hours 51.4’

Declination: +11° 49’

Do open clusters like M67 lead to radio telescope-building, IPad-loving civilizations? What’s the early history of our solar system, and how typical is it? Should the mantra for searching for life in the universe be “follow the metals?” Heady stuff to ponder as you show off this unassuming cluster with a fascinating tale to tell!


  1. [...] event trigger the collapse and formation of the inner rocky planets? Could we someday pinpoint our old birth cluster and brethren somewhere in the Milky Way galaxy [...]

  2. [...] in this regard.  Located in the constellation Cancer the Crab, M67 could also possibly be the birthplace of our Sun. Many of the stars in M67 exhibit a spectral signature of chemical composition very [...]

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