Credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond
(STScI and Pennsylvania State University), M. Dominik 
(University of St. Andrews), and Digitized Sky Survey
(STScI/AURA/UKSTU/AAO)

The nearest star  to our Sun, the red dwarf Proxima Centauri, is  on a course
for a rare conjunction  with two  distant background  stars.   This alignment
will offer  astronomers  a  unique  opportunity  to look for planets orbiting
close to Proxima Centauri. In addition, astronomers will be able to precisely
measure the mass of this isolated red dwarf.  Observations from NASA's Hubble
Space Telescope were used  to plot the star's trajectory across  the sky with
enough precision that astronomers  can predict  two "close encounters" of the
stellar kind with Proxima Centauri. The red dwarf passes nearly in front of a
20th-magnitude background star  in October 2014 and  a 19.5-magnitude star in
February 2016. The warping of space by Proxima Centauri's gravitational field
will cause the image of each star  to be very slightly offset from their real
positions on the sky.   The amount of offset can  be used  to measure Proxima
Centauri's mass — the greater the offset, the greater the mass of Proxima. If
Proxima Centauri has  a planet, it  may produce a small second position shift
due to the companion planet's gravitational field.

The stars will shift very slightly  in their apparent position,  an estimated
0.5 milliarcsecond and 1.5 milliarcseconds, respectively.   (A milliarcsecond
is the angular width  of  a  nickel  in  Honolulu, Hawaii, as viewed from the
distance of New York City.)   Hubble can  make  measurements as  small as 0.2
milliarcsecond.   The  European  Space  Agency's Gaia space telescope and the
European Southern Observatory's  Very Large Telescope on Mt. Cerro Paranal in
Chile may be able to make comparable measurements.  
These so-called microlensing events will last from a few hours to a few days.

Because Proxima Centauri is so close  to Earth, the area of sky warped by its
gravitation field  is larger than  for more-distant stars, and this makes the
observations easier to look for shifts in apparent stellar position caused by
this effect.

So far  all  attempts  to detect  planets around  Proxima Centauri  have been 
unsuccessful. Radial velocity and astrometric observations, which can measure
the gravitational wobble  of  a star due to  the pull of an unseen companion,
have not detected the presence of planets.  Astrometric observations rule out
giant planets with anything larger than  80 percent  of Jupiter's mass with a
period of less than 1,000 days. Likewise radial velocity observations exclude
the presence  of any planet the mass  of Neptune or  larger within an orbital
radius equal to  Earth's orbit (assuming the orbit is not face-on to Earth or
similarly inclined at a steep angle).  Nor have planets been found transiting
across the face  of the  star, which would happen  if the planet's orbit were
edge-on to Earth.   But now, by looking for microlensing effects during these
rare stellar alignments, smaller terrestrial  planets around Proxima Centauri
could be detected.

A team lead  by  Kailash Sahu  of  the  Space  Telescope  Science  Institute, 
Baltimore, Md. first searched 5,000 stars in the Luyten Half-Second catalogue
of stars that have a high rate of angular motion across  the sky for possible
alignment events.   "Proxima Centauri's trajectory turned out to offer one of
the most interesting opportunities because  of its extremely close passage to
the two stars," says Sahu.  Because it is so close to Earth, Proxima Centauri
also moves across  the sky at  a comparably high angular rate.   It crosses a
piece of sky with the apparent width of the full Moon every 500 years.

Red dwarfs are the  most common class  of stars in our Milky Way galaxy — any
such star that has ever  been born is still  shining today, because they live
longer  than   the  age  of  the  universe.   Therefore,  getting  a  precise
determination  of  mass  is critical  to understanding  a star's temperature,
diameter, intrinsic brightness, and longevity.

For every star like our Sun, there are approximately  10 red dwarfs in space.
Because lower-mass stars tend  to have smaller planets, red dwarfs  are ideal
places to go hunting for Earth-sized planets.

At a distance  of 4.2 light-years  from Earth,  Proxima Centauri is  just 0.2
light-years from the more-distant binary star Alpha Centauri A+B. These three
stars are considered part  of the triple-star system, though Proxima Centauri
evolved in isolation from  the two  Sun-like companion stars.  This paper has
been submitted to  the Astrophysical Journal and Dr. Sahu is presenting these
results at the  meeting of the American Astronomical Society in Indianapolis,
Indiana.

CONTACT
Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu