Image of the Week

Close encounters of the stellar kind


  The closest encounters with the Sun in space and time. The open circles show the expected perihelion (closest approach) distance vs. the expected perihelion time for each star. The error bars mark the 5% and 95% quantiles of the distributions for each star, which together form a 90% confidence interval on the estimate. Negative times are in the past; positive times are in the future.  

Stars are in constant relative motion, and inevitably, some will come close to the Sun. But how close? And what effect (if any) might they have? A sufficiently close encounter could perturb the orbits of comets in our Oort cloud, a primordial reservoir of billions of comets in the outer solar system extending to perhaps 0.5 pc (100 000 AU) from the Sun. Gravitational perturbations could push these comets onto orbits which send them into the inner solar system, where they could in principle collide with the Earth. Almost 200 impact craters have been identified on the Earth with ages of up to two billion years (many more craters have presumably long eroded or not yet been found). Some of these craters may be the result of impacts by such comets, and a large impact would have serious consequences for life on Earth.

Although Earthlings have nothing to worry about when compared to home-grown risks, it is nonetheless interesting to investigate how close stars have come - or will come - to the Sun. This can be done by measuring the current positions and velocities of stars, then tracing their orbits forwards and backwards in time within the gravitational potential of the Galaxy. Large-scale investigations like this only became possible with the advent of the Hipparcos catalogue in the late 1990s, a particularly notable study being that of Joan Garcia-Sanchez and colleagues in 2001.

Now, in a paper to appear in Astronomy & Astrophysics, Coryn Bailer-Jones of the Max Planck Institute of Astronomy in Heidelberg has tried to improve upon earlier studies. He uses the 2007 Hipparcos reduction by Floor van Leeuwen, adds more recent radial velocity measurements, and performs a comprehensive uncertainty analysis (by using Monte Carlo sampling to infer the probability distribution over the close encounter times and distances). Analysing some 50,000 Hipparcos stars, he confirms a number of previously discovered encounters, but also discovers several more. The above figure shows the closest encounters found in this study. For the full set of encounters, see the figures in the original paper. The closest encounter is apparently Hip 85605, which is found to have a 90% chance of approaching to 0.04 and 0.20 pc between 240 and 470 kyr from now (90% Bayesian confidence interval). However, as he stresses in the paper and on the FAQ on his website, the astrometric data for this star remain uncertain, so this result may be an artefact. The closest encounter with more reliable data is the rather unremarkable K dwarf, GL 710, calculated to encounter in 1.30-1.48 Myr at a distance of 0.10 to 0.44 pc (90% confidence intervals). The exact effect of such an encounter still needs to be modelled.

The results are of course limited by the available data. These will be greatly extended by Gaia, which will provide more accurate astrometry to fainter and more distant stars. Provided that reliable radial velocities can also be obtained, this will improve the space volume completeness and extend the time horizon of encounter studies, allowing also a more reliable statistical evaluation of the encounter rate than was possible with Hipparcos. Closer encounters await discovery.

credits: Coryn Bailer-Jones

[Published: 06/01/2015]


Image of the Week Archive

19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
Please note: Entries from the period 2003-2010 are available in this PDF document.