Solar Siblings

When our Sun formed 4.6 billion years ago, it almost certainly was a member of an open star cluster. Over several hundred million years, most of the stars in this cluster would have dissipated. Is there any hope, then, of finding some of our solar siblings?

I ran a query against the Gaia DR3 database to find stars with radial velocities and proper motions that are zero, within the measurement uncertainties. In other words, their space motions appear to be similar to that of the Sun. Could some of these stars be our long lost solar siblings?

First, some caveats.

  • 4.6 billion years is a lot of time, and dynamical evolution may lead to solar siblings no longer having comparable space motions to the Sun.
  • Error bars for the radial velocities, proper motions, and distances of many of these stars are large enough that subsequent more precise measurements may show that they are not co-moving with the Sun.
  • Though radial velocities are not affected by increasing star distance, proper motions are; therefore, proper motion in right ascension and declination will approach zero with increasing stellar distance
  • Some co-moving stars will be coincidental, especially if they are at large distances

I found 230 candidate stars in Gaia DR3 that appear to be co-moving with the Sun. They are listed in the table below.

Gaia DR3 Zero Space Motion

wdt_ID wdt_created_by wdt_created_at wdt_last_edited_by wdt_last_edited_at Gaia DR3 SOURCE_ID Other Catalog RA (2016) Dec (2016) G Mag Distance (ly)
1 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 5534600793005666944 TYC 7663-2637-1 08 05 30 - 40 05 11 10.63 2,100
2 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 4044381556633823232 HD 321719 18 25 18 - 34 39 16 10.91 3,776
3 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 5933186123279263872 TYC 8323-81-1 16 15 34 - 52 29 35 11.30 2,930
4 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 4069457877771166464 18 00 01 - 22 47 10 11.33 5,673
5 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 5926323972473953792 TYC 8349-1491-1 17 19 13 - 50 14 57 11.85 999,999
6 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 1816548038377615872 TYC 1639-1018-1 20 22 27 + 20 06 07 11.86 1,208
7 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 3403073120299336960 UCAC4 557-018920 05 44 22 + 21 14 45 12.00 1,720
8 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 5316984970605614208 08 46 50 - 54 57 33 12.08 1,192
9 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 2224937958644193920 V898 Cep 22 38 02 + 67 27 58 12.19 1,998
10 do18559252 30/04/2024 09:51 AM do18559252 30/04/2024 09:51 AM 4103489613769523712 18 42 09 - 14 55 00 12.21 2,988
Gaia DR3 SOURCE_ID Other Catalog RA (2016) Dec (2016) G Mag Distance (ly)

Please note that a distance of 999,999 ly (light years) indicates a Gaia parallax that is negative, meaning that the star is so far away that a reliable parallax cannot be measured. In other words, it is zero. Also, the farther away the star is, the more uncertainty there is in the distance.

19 of these 230 stars are bright enough, important enough, or lucky enough to have entries in the SIMBAD database. The nearest of these is TYC 8312-3134-1 which is 518 ly away in the constellation Norma.

We can do a simple BOTEC to determine how fast TYC 8312-3134-1 would have to be moving relative to the Sun to travel 518 ly in 4.6 Gyr. The answer is just 0.03 km/s = 30 meters/second. This is much less than the typical space motion of stars in the solar neighborhood relative to the Sun, which is on the order of many kilometers per second. It is therefore completely plausible that solar siblings could now be at a distance of at least 500 ly and even many times further than that.


SELECT TOP 2000   gaia_source.source_id,gaia_source.ra,gaia_source.dec,gaia_source.parallax,gaia_source.pmra,gaia_source.pmdec,gaia_source.ruwe,gaia_source.phot_g_mean_mag,gaia_source.bp_rp,gaia_source.radial_velocity,gaia_source.radial_velocity_error,gaia_source.phot_variable_flag,gaia_source.non_single_star,gaia_source.has_xp_continuous,gaia_source.has_xp_sampled,gaia_source.has_rvs,gaia_source.has_epoch_photometry,gaia_source.has_epoch_rv,gaia_source.has_mcmc_gspphot,gaia_source.has_mcmc_msc,gaia_source.teff_gspphot,gaia_source.logg_gspphot,gaia_source.mh_gspphot,gaia_source.distance_gspphot,gaia_source.azero_gspphot,gaia_source.ag_gspphot,gaia_source.ebpminrp_gspphot
FROM gaiadr3.gaia_source 
WHERE (gaiadr3.gaia_source.radial_velocity-gaiadr3.gaia_source.radial_velocity_error <= 0)
  and (gaiadr3.gaia_source.radial_velocity+gaiadr3.gaia_source.radial_velocity_error >= 0)
  and (gaiadr3.gaia_source.pmra-gaiadr3.gaia_source.pmra_error <= 0)
  and (gaiadr3.gaia_source.pmra+gaiadr3.gaia_source.pmra_error >= 0)
  and (gaiadr3.gaia_source.pmdec-gaiadr3.gaia_source.pmdec_error <= 0)
  and (gaiadr3.gaia_source.pmdec+gaiadr3.gaia_source.pmdec_error >= 0);

Epoch and Equinox

We use the term epoch (of a given date) to refer to the actual measured coordinates of a star that takes into account precession, nutation, and proper motion. The term equinox means that the coordinates have been precessed to a given date, but that other factors affecting a star’s position have not been applied. So, equinox 2000.0 is not the same as epoch 2000.0.

Example: Barnard’s Star

Epoch 2000.0 coordinates: α = 17h 57m 48.49803s, δ = +4° 41′ 36.2072″ (the actual position of Barnard’s Star at 0h UT on January 1, 2000, accounting for precession, nutation, and proper motion)

Equinox 2017.1 coordinates: α = 17h 58m 39.20689s, δ = +4° 41′ 33.5614″ (coordinates have been precessed from epoch 2000.0 above to today’s date, but nutation and proper motion have not been applied)

Epoch 2017.1 coordinates: α = 17h 58m 37.85s, δ = +4° 44′ 37.8″ (the actual position of Barnard’s Star on January 19, 2017, accounting for precession, nutation, and proper motion)

Sometimes, the epochal coordinates are further adjusted to account for aberration and atmospheric refraction.  The latter tends to “lift” stars towards the zenith—the closer to the horizon the greater the lift.

Intergalactic Stars

Did you know that a few percent of all stars are traveling alone through intergalactic space, no longer a part of any galaxy?  Gravitational interactions between stars or between stars and black holes can occasionally accelerate a star to galactic escape velocity so that it is thrown (eventually) into intergalactic space.  When the star first enters intergalactic space, the view of your home galaxy would be pretty remarkable, but eventually (eons later, of course) there would be very few naked eye objects in your night sky. Just moons and planets, meteors, aurora, comets, the zodiacal light, and maybe a galaxy or two. Anything else would require a telescope.  And an observer, of course.

The first evidence for intergalactic stars came from the detection of diffuse light between galaxies (Zwicky 1952).  Much later, intergalactic planetary nebulae were detected in the Fornax galaxy cluster (Theuns & Warren 1997).  More recently, intergalactic red giant stars have been detected in the Virgo galaxy cluster using the Hubble Space Telescope (Ferguson et al. 1998).

The Fornax cluster lies about 62 million light years distant, and the Virgo cluster 54 million light years distant.  Have any intergalactic stars been detected near our Milky Way galaxy?  Brown et al. (2005) discovered the first hypervelocity star, SDSS J090745.0+024507, a 20th-magnitude star in the constellation Hydra.  Though it is just 160,000 light years from the center of our galaxy, it is moving away from the Galactic center at an astonishing radial velocity of 709 km/s.  Even though this one-dimensional radial velocity1 is only a lower limit to the star’s true 3D space motion, it is far and away fast enough to escape our Milky Way galaxy altogether.  Gaia will probably be able to measure this runaway star’s proper motion in right ascension and declination, thus allowing a determination of the true space velocity of SDSS J090745.0+024507 relative to the Galactic center.

Several more hypervelocity stars have been discovered since 2005.  One of them, US 708, a 19th-magnitude white dwarf in Ursa Major, is exiting our galaxy at a velocity of at least 1200 km/s!  This makes it the fastest on record (Geier et al. 2015).

1The observed one-dimensional radial velocity as seen from Earth is corrected for the Earth’s rotation and motion around the Sun, and the Sun’s motion around the center of the Milky Way galaxy to determine the galactocentric radial velocity.

Brown, W. R., Geller, M. J., Kenyon, S. J., Kurtz, M. J. 2005, ApJ, 622, L33
Ferguson, H. C., Tanvir, N. R., & von Hippel, T. 1998, Nature, 391, 461
Geier, S., Fürst, F., Ziegerer, E., et al. 2015a, Science, 347, 1126
Theuns T., Warren S. J., 1997, MNRAS, 284, 11
Zwicky F., 1952, PASP, 64, 242