Thursday 21 July 2022

Looking for close encounters between the Sun and other stars.

Close encounters between stars have the potential to alter the evolution of planetary systems. This is particularly true in the early stages of their development, when planets are forming from circumstellar discs, a process which could easily be disrupted by an encounter with another star, but still applies later in a system's history, as most stars are thought to be surrounded by a loosely bound body of comets similar to our Oort Cloud, from which bodies could be disrupted sending them into the inner stellar system, potentially leading to impacts on the surface of planets, with a profound impact on any life their. A close star could also potentially go supernova, with even more devastating effect.

Clearly this makes close encounters between the Sun and other stars of great interest to scientists studying the history of the Solar System, who have speculated about such encounters for many years. Serious attempts at calculating the frequency of such close encounters has been possible only since 1997, when the Hipparcos star catalogue was produced, which for the first time included the parallaxes of over 118 000 stars. Two decades this became a much more realistic task with the production of the much more detailed Gaia catalogue, the development of software capable of analysing such large amounts of data, and the wider availability of computers capable of running such software.

Since its original release Gaia has been updated twice, with the current catalogue containing information on the movements of about 34 million bright stars.

In a paper published on the arXiv database at Cornell University on 14 July 2022, and accepted for publication in The Astrophysical Journal Letters, Coryn Bailer-Jones of the Max Planck Institute for Astronomy uses data from the latest version of the Gaia catalogue to produce a list of stars likely to have passed within 1 parsec (3.26 light years) of the Sun within about 6 million years of the current date. 

Bailer-Jones notes that some previous attempts at producing lists of close encounters have used limits greater than 1 parsec, but considers that since our current closest neighbour, the Alpha Centauri system, is only 1.3 parsecs (4.24 light years) away, 1 parsec seems a more useful upper limit. The Oort Cloud is currently thought extend to somewhere between 0.25 and 0.5 parsecs (0.82 and 1.63 light years) from the Sun, so stars coming closer than 1 parsec might be presumed to be starting to have some impact upon this cloud. The degree of error present in the Gaia catalogue makes calculation of encounters more than 6 million years into the past or future unreliable. Finally, the Gaia catalogue only includes stellar bodies larger than 0.12 times the mass of the Sun, so it is likely that other encounters with smaller stars (which are numerous) will have been missed.

Bailer-Jones's eventual list includes 61 stars, all currently within 380 parsecs (1329 light years) of the Sun, and with an average distance of 100 parsecs (326 light years), although he notes that this cannot be seen as a definitive list, and is likely to contain inaccuracies, particularly in the case of binary stars, which tend to have more complicated movement patterns, harder to calculate, and White Dwarf stars, which are notoriously hard to track. The list also includes two very young pre-main sequence stars, which may not have existed at all at the times when they theoretically came close to the Sun. 

Perihelion (encounter) times and distances for the 61 stars that have a median perihelion distance below 1 parsec. Negative times indicate past encounters, positive times future ones. The circles/squares show the median of the perihelion time and distance distributions computed from the 1000 data resamples (surrogates); the error bars show the 5th and 95th percentiles. Circles denote good encounters; squares denote the questionable encounters. The colour of each point indicates the median encounter velocity; those faster than 100 km per second are white. Bailer-Jones (2022).

Of the 61 stars for which an encounter closer than 1 parsec could be predicted, only 13 are predicted to come within 0.5 parsecs (1.63 light years).

The first star on Bailer-Jones's list is Gliese 710 (DR3 4270814637616488064 on the Gaia catalogue, and HIP 89825 on the Hipparcos catalogue). Gliese 710 is a K Class Orange Dwarf Star with approximately 70% of the Sun's mass, currently 19.1 parsecs (62.3 light years) from our Solar System in the constellation of Serpens. This star was first identified as likely to have a future close encounter with our Solar System in 1999, when it was calculated that it could potentially reach 0.34 parsecs from the Sun in 1.36 million years time. Since this first analysis Gliese 710 has been the subject of considerable interest to astronomers, and the time and distance of such a close encounter has been revisited several times. Bailer-Jones's new analysis is the most detailed yet, and calculates that the star may come to 0.064 parsecs (0.21 light years, or 13 200 astronomical units) in 1.29 million years. This is too distant to have any significant effect on the orbit of Pluto, but could potentially influence comets in the outer Oort Cloud quite severely. 

The second star on the list is HD 7977 (DR3 510911618569239040 on the Gaia catalogue), a G Class Yellow Dwarf Star with about 1.2 times the mass of the Sun, currently 75.6 parsecs (246.7 light years) from the Solar System in the constellation of Cassiopeia. This was first identified as likely to have a close encounter with the Sun by Bailer-Jones in 2018, when he calculated that it reached 0.43 parsecs (1.4 light years) from the Sun about 2.76 million years ago. The new data lowers this distance considerably, to 0.064 parsecs (0.21 light years).

The third star on the list is UPM J0812-3529 (DR3 5544743925212648320 on the Gaia catalogue), which was first identified as a nearby White Dwarf Star in 2018, but which has not previously been included in any study of potential close encounters. Bailer-Jones calculates that this star could reach 0.11 parsecs (0.36 light years) from the Sun in 29 000 years time. However, as previously noted, the distance and velocity of White Dwarf stars is notoriously hard to calculate, and the assumed timing and distance of approach for UPM J0812-3529 is based on current estimates that the star is 11.2 parsecs (36.5 light years) away, and travelling at a speed relative to the Sun of 374 km per second, both of which are uncertain, with the later being suspiciously large. This said, Bailer-Jones points out that a lower relative speed could lead to a closer encounter.

The fourth star on the list is UCAC4 689-035468 (DR3 213090546082530816 on the Gaia catalogue), another star identified as likely to have a close encounter with the Sun by Bailer-Jones in 2018, but for which the distance of this approach has been considerably reduced, from 1.44 parsecs (4.7 light years) in 2018 to 0.248 parsecs (0.81 light years) now, with such an encounter happening in 8.3 million years.

The fifth star on the list is identified on the Gaia catalogue as DR3 5571232118090082816, and is predicted to have come to 0.26 parsecs (0.85 light years) from the Sun 1.16 million years ago.

The sixth star on the list CD-25 8217 (DR3 5469802896279029504 on the Gaia catalogue), is also the latest possible encounter on the list, potentially reaching 0.35 parsecs (1.14 light years) from the Sun in 11.12 million years time. This is also predicted to be the slowest encounter on the list, both factors arising from its low relative velocity, approaching the Sun at only 3.2 km per second.

The seventh star on the list, identified as DR3 3372104035275483392 in the Gaia catalogue, is predicted to reach 0.36 parsecs (1.17 light years) from the Sun in 1.71 million years time.

The eighth star on the list, identified on the Gaia catalogue as DR3 3207963476278403200 is predicted to have reached 0.36 parsecs from the Sun 514 000 years ago, although Bailer-Jones identifies this as somewhat suspicious; the star is apparently above the main sequence, with a suspiciously high relative velocity and a fainter star behind it. This may in fact be a pre-main sequence star with a close companion, in which case the assumptions about it's movement would be quite wrong.

The ninth star on the list, identified as DR3 3106500096597409792 on the Gaia catalogue, is predicted to have reached 0.37 parsecs (1.19 light years) from the Sun 8.7 million years ago.

The tenth star on the list, DR3 4763293626627587840 is predicted to reach 0.45 parsecs (1.47 light years) from the Sun in 1.54 million years time.

The eleventh star on the list, DR3 929788371508812288 is predicted to have come to 0.46 parsecs (1.51 light years) of the Sun 364 000 light years ago.

The twelfth star on the list, DR3 6913732624445112832 is predicted will reach 0.46 parsecs from the Sun in 2.9 million years.

The thirteenth star on the list, DR3 3118526069444386944 is a young stellar object previously calculated to have reached 1.03 parsecs (3.36 light years) from the Sun, a distance now reduced to 0.49 parsecs (1.6 light years). However, this body is now considered to be a close binary star, and Bailer-Jones considers that this makes calculations of its velocity highly unreliable.

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