Stars that approach within one parsec of the Sun:
New and more accurate encounters identified in Gaia Data Release 3

C.A.L. Bailer-Jones

Close encounters of stars to the Sun could affect life on Earth through gravitational perturbations of comets in the Oort cloud or exposure to ionizing radiation. By integrating orbits through the Galactic potential, I identify which of 33 million stars in Gaia DR3 with complete phase space information come close to the Sun. 61 stars formally approach within 1 pc, although there is high confidence in only 42 (two thirds) of these, the rest being spurious measurements or (in) binary systems. Most of the stars will encounter within the past or future 6 Myr; earlier/later encounters are less common due to the magnitude limit of the Gaia radial velocities (RVs). Several close encountering stars are identified for the first time, and the encounter times, distances, and velocities of previously known close encounters are determined more precisely on account of the significantly improved precision of Gaia DR3 over earlier releases. The K7 dwarf Gl 710 remains the closest known encounter, with an estimated (median) encounter distance of 0.0636 pc (90% confidence interval 0.0595-0.0678 pc) to take place in 1.3 Myr. The new second closest encounter took place 2.8 Myr ago: this was the G3 dwarf HD 7977, now 76 pc away, which approached within less than 0.05 pc of the Sun with a probability of one third. The apparent close encounter of the white dwarf UPM J0812-352 is probably spurious due to an incorrect RV in Gaia DR3.

The plots below shows the location of the star at perihelion (closest encounter) relative to the Sun in Galactic Cartesian coordinates. x points away from the Galaxy centre in the plane of the Galaxy in the phi=0 direction (the longitude of the Sun); z points perpendicularly out of the Galactic plane towards the north Galactic pole; y completes the right-handed coordinate system (so it points in the opposite direction of the rotation of the Galaxy). If the Sun was at the centre of the Galactic plane then the x axis would pass through the Sun, but in the model used in the paper the Sun is (at the present time) above the Galactic plane. Thus the xy-plane is the Galactic plane, and the xz- and yz-planes are the two "side" views.

Each point is one surrogate star, i.e. a random realization of the star's 6D coordinates (drawn from the uncertainties in position and proper motion) which has been integrated through the Galactic potential to perihelion. The actual position of the Sun relative to the Galactic centre is slightly different for each surrogate because the time of the encounter is different for each surrogate. So each encounter occurs in a frame which is slightly offset from the other, but all with the same orientation, which are then superimposed here.

The plot above is for Gl 710 (Gaia DR3 4270814637616488064), the closest encountering star. The orange cross is the Sun (origin), each black point a surrogate. The blue arc is a cricle of radius equal to the median encounter distance, in this case 0.0636 pc. In the Galactic plane (left), we see that the surrogates form a long thin distribution poined at the Sun. In the other two projections the surrogates are more circular. Which projections are more elongated and which more circular depends on the fractional uncertainty of the velocity of the star in the x, y, z directions, which are not parallel to the proper motion and radial velocities (as those are defined in spherical polar coordinates relative to the Sun). The "side" projections show that the surrogates are all passing above the Sun.

The plot above is for HD 7977 (Gaia DR3 510911618569239040), the second closest encounter. The median encounter distance (radius of blue circle) is 0.0641 pc. Here, the projected positions of the surrogates in the Galactic plane (left) are on both sides of the Sun. This plot illustrates the point discussed in the last paragraph of section 3 of the paper: If we decided to average the 2D positions in this plot, we would get an average position very close to zero, because the surrogate pass on both sides of the Sun, and so many positive and negative quantities average out: we get = 0.0049 pc and = 0.0046 pc (using the mean; with the median we get 0.0058 pc and 0.0052 pc respectively). If we then computed a distance from these average positions we would get sqrt(^2 + ^2) = 0.0067 pc (0.0078 pc using the medians). But clearly this is not a good measure of the average distance of the surrogates, , which in this plane is 0.0212 pc (or 0.0178 pc if we take the median). In other words, to compute the average distance of a populations we must compute the average of the individual distances, not the distance of the average position. Of course, in reality the encounter takes place in three-dimensions, not two, and we see from the two "side" projection plots that most surrogates pass "under" the Sun. The blue circle, remember, shows the average (median) of the three-dimensional distances of the surrogates.