The Birth Environment of the Sun

Chemical tagging is a scientific technique that compares the elements in a star to the elements in other stars in order to determine whether they originated in the same star cluster. The most likely birth environment of the sun, based on chemical tagging, is old open cluster M67, which also has a time of origin consistent with that of the sun. There are a few scientific problems associated with the origin of the sun in M67. First, the sun would have needed to exit this dense cluster quickly in order for the planets to form. Second, it is highly unlikely that the sun would be in its current orbit in the galaxy if it originated in M67.

Just as people learn about their heritage with genealogical research methods, scientists investigate the origin of stars with chemical tagging. Determining the birth environment of the sun is a complex problem that includes first searching for solar twins (color, mass, and age), using chemical tagging to compare the abundances of elements in stars with other stars and clusters in the galaxy. They also use lithium concentration, stellar internal mass circulation dynamics, and other methods to determine the age of stars. Finally, if there is a match, scientists calculate the orbital dynamics of stars and clusters in the galaxy to determine whether a star might have originated in a cluster. They also look at the gravitational dynamics within clusters to determine the possible exit velocity of stars from that cluster. Finally, scientists calculate the possibility that disk and planetary systems would survive gravitational interactions and radiation from stars in the cluster and the potential exit velocity from the cluster.

The science of chemical tagging is comparable to DNA ancestry techniques: scientists compare the spectral signature (absorption lines) of stars to each other in order to determine whether stars might have originated in the same cluster. Scientists compare stars of similar mass since the depletion and circulation of elements in stars is a function of mass. One example of chemical tagging is the study of the incorporation of dwarf galaxies from the Local Group into the Milky Way. In general, dwarf galaxies leave behind streams of stars, and scientists determine which stars used to be part of the dwarf galaxy by looking at their "fossil signatures."

Of all the chemical tagging research, scientists are most interested in finding the birthplace of the sun. Thus, scientists have been searching for solar twins since the 1970s.[1] Scientists discovered several solar twin candidates, having the same mass and similar ages as the sun; however, most stars have systematic deviations in element concentrations from the sun.[2] One solar twin in the old open M67 cluster, M67-1194 (Figure 3‑21), has a spectral signature that is the same as sun.[3] [4] [5] [6] The sun is also similar to other stars in old open cluster M67 (Messier Object 67), but none as close as M67.

The estimated date of formation of M67 correlates with the age of the sun. Yadav et al. analyzed the ages of stars in the M67 cluster and found that M67 formed between 3.5 and 4.8 billion years ago, which is 4.15 +/- 0.65. [7] This means that both age and chemical profile allow for the origin of M67-1194 and the sun in the same molecular cloud. Onehag stated,

“We find M67-1194 to have stellar parameters indistinguishable from the solar values, with the exception of the overall metallicity which is slightly super-solar ([Fe/H] = 0.023 ± 0.015). An age determination based on evolutionary tracks yields 4.2 ± 1.6 Gyr. Most surprisingly, we find the chemical abundance pattern to closely resemble the solar one, in contrast to most known solar twins in the solar neighborhood. We find that radiative dust cleansing by nearby luminous stars may be the explanation for the peculiar composition of both the Sun and M67-1194, but alternative explanations are also possible. The chemical similarity between the Sun and M67-1194 also suggests that the Sun once formed in a cluster like M67” [8]

Recently, Julia Ahlvind wrote a project report in an advanced physics project class under the direction of Andreas Korn and Bengt Gustafsson, who were the 2nd and 3rd authors on Onehag's paper ,in which she estimated the age of the M67 cluster with an isochrone method as 4.56 +/- 0.44 Ga, where the average is the same as the age of the sun. [9] FULLTEXT01.pdf (diva-portal.org)

The elements in the sun and the stars in M67 are different than the elements in the overall population of solar twins in the galaxy. Based on the chemical characteristics of the known population of solar twins, the probability that M67-1194 are typical solar twins is less than 5%, which means that M67-1194 and the sun are from a different population (odd origin) than the typical solar twin in the galaxy. The primary difference between most solar mass stars (solar twins) and the sun is that the majority have a lower fraction of volatile elements (i.e. nitrogen, oxygen, carbon) and a higher fraction of refractory elements (mineral dust) than the sun.

“Meléndez et al. found that for almost all their stars, the volatile elements are relatively less abundant than in the Sun, while the refractory elements are somewhat enhanced in the stars. This effect is seemingly not present in the M67 twin, at least not to the same extent. It is thus seen (cf. Fig. 7) that the abundance profile of M67-1194 is more similar to the Sun than for all the Meléndez et al. twins, with one or two exceptions. We perform a simple statistical test, essentially judging the probability that all black dots in Fig. 7 just by chance fall above the red dashed line for T_cond >  1400 K, and below it for T_cond <  1400 K. We find that the probability for M67-1194 to be drawn from the same sample of stars as the majority of the Meléndez et al. twins is less than 5%. On the other hand, the errors in the analysis are fully compatible with solar abundance ratios for the M67 star. This astonishing result will be discussed further below.” [10]

“The similarity of the age and overall composition of the Sun with the corresponding data of M67, and in particular the agreement of the detailed chemical composition of the Sun with that of M67-1194, could suggest that the Sun has formed in this very cluster.” [11]

Although the sun and M67-1194 must have formed in similar environments, the different galactic orbits of the sun and M67 argues against a common origin.

“The chemical abundance pattern of M67-1194 closely resembles the solar one, in contrast to most known solar twins. This suggests similarities in the formation of M67-1194 and the Sun. A common origin of both stars as members of M67 is conceivable, albeit not likely, considering their different Galactic orbits. If the chemical abundance pattern reflects environmental effects, then the Sun was likely born in a cluster similar to M67.” [12]

Onehag proposed three possible explanations for the systematic difference between the sun/M67-1194 and other solar twins. The first was a thin solar convection zone. The second was pollution by the infalling disk. The third was that the molecular cloud/s that formed the sun and M67-1194 were impacted by radiation fields from massive stars in the cluster, which “pushed dust grains out of the cloud.” She favored the latter option.

“In the standard picture of stellar evolution with an early fully-convective phase, dust cleansing by nearby luminous stars seems to have affected both the Sun and M67-1194.” [13]

The science of chemical tagging is a developing field with improvements in instrumentation and techniques. Liu conducted a more extensive analysis than Onehag and analyzed 28 elements in the sun and M67-1194 and M67-1315. They found that the concentrations of all 28 elements were the same in M67-1194 and the sun, and they stated, "M67-1194 is also found to have identical chemical composition to the sun, confirming its solar twin nature." [14] Figure 3‑22 plots spectral responses of the sun and M67-1194 at the same wavelengths. The correlation coefficient, R², is 0.9972. This method of plotting and calculation of the correlation coefficient is misleading because it exaggerates the actual correlation due to the alignment of responses; however, it can be compared to the alignment of other solar twins with the sun and calculation of the standard error (average difference) is also possible.

The standard error in Figure 3‑22 is 1.34, which means that the average difference between the solar spectral lines and M67-1194 is 1.34. Since the scale of the spectral responses is in the range of 100, the standard error indicates that the average deviation between the sun and M67-1194 spectral responses is approximately 1%. This is a low number and is indicated by the closeness of the points to the regression line (dashed line). The slope of the line in Figure 3‑22 is almost precisely 1.0 (0.9983), which means that the solar spectra and M67-1194 have the same general magnitude (0.2% difference). While this is also somewhat deceptive since similar values are aligned, it is not the same in other solar twins for example, M67-1315 is another solar twin in M67. The alignment of the solar and M67-1315 spectra is shown in Figure 3‑23. The standard error is 3.21, which means that the average deviation from the regression line is 3 times higher than for the sun and M67-1194. The slope of the regression is 0.93, which means that the average magnitude of the spectral responses in M67-1315 is approximately 7% less than in the sun. Likewise, Liu stated that M67-1315 is 0.06 dex lower than the sun. Liu stated that one reason for the difference between the sun and M67-1315 is that it is slightly cooler than the sun whereas M67-1194 is the same temperature.

Because M67 has survived in the galaxy for approximately 4 billion years, astronomers think that M67 was originally a compact starburst cluster, such as the cluster of stars in Figure 3‑24 (NGC 3603), which has tens of thousands of stars. Starburst clusters are very dense. Their gravitational fields exceed the gravitational field of the galaxy. Thus, they hold together as they orbit through the disruptive forces of the galaxy. Starburst clusters break apart after billions of years rather than the few hundred million years of “leaky” open clusters. On the other hand, the gravitational field of smaller or more dispersed clusters is exceeded by the gravitational field of the galaxy, and galactic forces break these clusters apart within a few hundred million years. Gravity within clusters is least intense in the outer sections of clusters, which is why galactic forces gradually tear stars in the outer part of clusters away from the cluster. Because starburst clusters are dense and have many large stars, they cannot form planetary systems such as the solar system.

The Sun and M67-1194 might have formed within a small molecular cloud such the Pillars of Creation (Banner) within a larger cluster such as in the Eagle Nebula. Stars that form in a remaining cloud such as the Pillars of Creation would have similar spectral signatures due to the relative uniformity of materials that are mixed in the smaller gas cloud. It is likely that radiation from large stars cleansed the cloud of dust, and supernovae explosions would have impregnated the cloud with their residue, as evidenced by Iron 60 in meteorites.

Holzer et al. investigated whether variation in elemental compositions in the stars in Messier 67 was linked to the formation of planets around the stars.[15] Disks flow into stars, and planets remove elements from the disk, thus altering the chemistry of the disk. Astronomers are also interested in the planetary systems in M67 because they want to learn about the formation of the planets in our solar system. They detect planets by looking a wobbles in the star caused by the gravity of the planet.[16] The planet orbiting star M67-1194 has a 6.9 day orbital period, a 0.24 eccentricity (not circular), and a mass 1/3 the mass of Jupiter. Astronomers call large planets with short orbital periods (close to their host star) hot Jupiters. Other planets in the M67 cluster have similar characteristics. Star M67-1514 also has a hot Jupiter. Evolved star S364 has a planet with a longer orbital period, 121 days, a mass 1.5 times the mass of Jupiter, and 0.35 eccentricity. This orbital period is approximately the period of the orbit of Mercury. Brucalassi et al. (2016) stated that the high frequency of hot Jupiters in M67 is possibly a function of interactions with nearby stars or even planets.[17] The fact that there are hot Jupiters orbiting M67 solar twins is odd since the solar system has several small and widely spaced planets and has no hot Jupiters near the sun. Even if the sun did not form in M67, chemical tagging evidence indicates that it formed in a similar cluster, which would not be conducive to the formation of widely spaced planets as in the solar system.

It is likely that there are only large planets close to the stars in M67 because planets have far more disruptions in starburst clusters due to frequent encounters with the gravitational fields of other stars in the densely packed cluster. The sun would have also had gravitational and radiative effects from large stars in the vicinity, which is why Richard Parker presented the following dichotomy concerning evidence of the proximity of the protosun to large stars and the presence of our widely spaced planets in the solar system.

(i) “Gas giant planets must form extremely quickly (within 1 Myr), and within 10 AU of the host star where the effects of external photoevaporation will be limited.

(ii) Alternatively, gas giant planets form exclusively in star-forming regions that do not contain massive stars. This latter point is in direct tension with meteoritic evidence suggesting that our own Solar System was in close proximity to massive stars as planets were forming.” [18]

Parker went on to describe the relationship between the sun and M67.

“Indeed, some authors have claimed one of the Messier objects, the M67 open cluster, may be the birth cluster of the Sun, based on its similar age and almost identical chemistry of the stars. We know from observations that planets are present around stars in this cluster including around Solar twins. As M67 is a relatively massive open cluster, it is likely that the young Sun may have experienced significant radiation fields that could have evaporated the gas in its protoplanetary disc. However, this also suggests that enrichment in 26Al and 60Fe could have occurred. If the Sun did originate in M67, it may have been ejected early on, so that subsequent interactions in the cluster did not disrupt the outer planets. Interestingly, attempts have been made to calculate when and where in time the Sun and M67 intersect, without much success, arguing that perhaps the Sun may have originated elsewhere (and the similarity in the chemistry of M67’s stars to the Sun could be chance). Alternatively, a collision with a GMC could have disrupted the orbit of M67 itself, making it impossible to rule out the Sun’s origin in this cluster." [19]

Suzanne Pfalzner analyzed the probability of the destruction of the sun’s circumsolar disk and planetary orbits in a starburst cluster. Using computer simulations, she determined that the probability of destruction is 100 percent (Figure 3 in the Pfalzner paper). She found that the probability of the solar system surviving, particularly with eight planets with nearly circular orbits, would be zero in a starburst cluster.[20] If the sun was in a starburst cluster such as M67 for any length of time, then it could not have formed the current set of planets.

In order to assess the possibility that the sun was ejected from M67 early in the life of the sun, Barbara Pichardo analyzed the path of M67 through the galaxy and compared it to the path of the sun in the galaxy. The M67 cluster oscillates much farther above and below the galactic plane than the sun. The sun moves approximately 250 light-years above and below the galactic plane. The average thickness of the galactic plane is 1,000 light-years or 500 light-years above and below the galactic plane. M67 moves approximately 1,000 light years above and below the galactic plain. Pichardo analyzed whether gravitational interactions with other stars in the cluster could have thrown the sun out of its orbit in M67 and into the sun’s current orbit in the galaxy.[21] Pichardo found that the sun could not have been ejected from M67 without disrupting the orbits of the planets. She thus concluded that the sun did not originate in M67.

There is an advantage to the sun’s galactic orbit over that of M67. If the sun did not remain within the galactic plane, then radiation from distant supernova explosions in the galaxy would be much more likely to destroy life on Earth because gas and dust in the plane intercept the radiation from these explosions. In contrast, M67 moves significantly above and below the plane of the Milky Way. Thus, life in M67 would be more susceptible to extinction from supernovae explosions than the solar system. The sun's orbit in the Milky Way Galaxy is advantageous for life in other ways. The sun is in a quiet location with relatively few stars in the Milky Way. It is also at the corotation distance in the galaxy. This distance results in the fact that the sun does not frequently move through the dense arms of the galaxy. Even among galaxies, the Milky Way Galaxy is particularly amenable to life since it has remained as a spiral galaxy for billions of years and is in a part of the universe with few large galaxies but many dwarf galaxies. All of these factors do not imply that the earth is unique in its capacity to support life, but we are in a location in the solar system, galaxy, and universe that is advantageous.

[1] Anna Onehag, Andreas Korn, Bengt Gustaffson, Eric Stempels, and Don Vandenberg, "M67-1194, an unusually sun-like solar twin in M67," Astronomy and Astrophysics, 528 (2011):

[2] Onehag, M67.

[3] G. Pace, L. Pasquini, and P. François, "Abundances of Four Open Clusters from Solar Stars," Astronomy & Astrophysics, 489:1 (2008): 412

[4] Luca Pasquini, K. Biazzo, P. Bonifacio, S. Randich, and L. Bedin, "Solar Twins in M67," Astronomy & Astrophysics, (2008). Accessed on September 5, 2012 at http://arxiv.org/abs/0807.0092v

[5] Onehag, M67.

[6] Liu, F., M. Asplund, D. Yong, J. Meléndez, I. Ramírez, A. I. Karakas, M. Carlos, and A. F. Marino. "The chemical compositions of solar twins in the open cluster M67." Mo. Notices of Royal Astronomical Society (2016):

[7] R. K. S. Yadav, L. R. Bedin, G. Piotto, J. Anderson, S. Cassisi, S. Villanova, I. Platais, L. Pasquini, Y.Momany, and R. Sagar, "Ground-based CCD astrometry with wide-field imagers II. A star catalog for M 67," Astronomy and Astrophysics, 484 (2008): 610.

[8] Onehag, M67

[9] Ahlvind, Julia. "Isochrone and chemical ages of stars in the old open cluster M67." (2021). Advanced Physics class project report. Uppsala University.

[10] Onehag, M67

[11] Onehag, M67

[12] Onehag, M67

[13] Onehag, M67

[14] Liu, Open cluster, M67

[15] Holzer, Parker H., Inese Ivans, Jessica Galbraith-Frew, Tim Anderton, and Apogee Team. "The Chemical Composition of Planet-Harboring Stars in M67." In APS April Meeting Abstracts. 2016.

[16] A. Brucalassi1, L. Pasquini, R. Saglia1, M.T. Ruiz, P. Bonifacio, L. R. Bedin, K. Biazzo, et al., Three planetary companions around M67 stars, Astronomy & Astrophysics manuscript no. letterM67. Accessed at < http://www.eso.org/public/archives/releases/sciencepapers/eso1402/eso1402a.pdf>

[17] Brucalassi, A., L. Pasquini, R. Saglia, M. T. Ruiz, P. Bonifacio, I. Leão, BL Canto Martins et al. "Search for giant planets in M67-III. Excess of hot Jupiters in dense open clusters." Astronomy & Astrophysics 592 (2016): L1

[18] Parker, Richard J. "The birth environment of planetary systems." Royal Society open science 7, no. 11 (2020): 201271.

[19] Parker, The birth environment.

[20] Susanne Pfalzner, “Early Evolution of the Birth Cluster of the Solar System,” Astronomy and Astrophysics (November 1, 2012), arXiv:1210.8255

[21] Barbara Pichardo, Edmundo Moreno, Christine Allen, Luigi R. Bedin, Andrea Bellini, and Luca Pasquini, "The Sun was Not Born in M67," The Astronomical Journal, 143:73 (2012): 9-10. Accessed September 5, 2012, doi:10.1088/0004-6256/143/3/73

Banner. Wide Field Camera 3 image of the Pillars of Creation, 2014. Credit: NASA, ESA.