Rare Stellar Alignment Helps in Hunt for Planets

The red dwarf Proxima Centauri, our sun’s nearest neighbor is heading for a rare conjunction with two distant background stars. First, Proxima Centauri will pass nearly in front of a 20th-magnitude star in October 2014. In 2016, the red dwarf will pass a 19.5-magnitude star. The alignment will allow astronomers to look for small planets orbiting close to Proxima Centauri. It will also provide data to precisely determine the mass of our nearest stellar neighbor.

"Proxima Centauri’s trajectory turned out to offer one of the most interesting opportunities because of its extremely close passage to the two stars."

– Kailash Sahu of the Space Telescope Science Institute, Baltimore, Md.

Some facts:

  • Proxima Centauri’s gravitational field will warp space slightly, making it look like the background stars are slightly offset from their true positions.
  • The first star is expected to shift by only 0.5 milliarcsecond.
  • The second star is expected to shift by 1.5 milliarcseconds.
  • Hubble can measure down to 0.2 milliarcsecond.
  • The European Space Agency’s Gaia space telescope and the European Southern Observatory’s Very Large Telescope might be able to make similar measurements.

How far away are we talking?

Proxima Centauri is the closest star to our solar system… but it is still 39,900,000,000,000 km away, or 4.22 light years.

A light year is the distance light travels in one year – it is equal to 9.46 x 1012 km. The stars Alpha Centauri A+B are about 4.35 light years away from us, meaning that Proxima Centauri is just slightly closer.

Proxima Centauri evolved separately from the binary star Alpha Centauri A+B. However, the three stars are considered a triple-star system.


Plot of the projected motion of the red dwarf star Proxima Centauri
Plot of the projected motion of the red dwarf star Proxima Centauri (green line) over the next decade, as plotted from Hubble Space Telescope observations. Because of parallax due to Earth’s motion around the Sun, the path appears scalloped. Because Proxima Centauri is the closest star to our Sun (distance, 4.2 light-years), its angular motion across the sky is relatively fast compared to much more distant background stars. This means that in 2014 and 2016 Proxima Centauri will pass in front of two background stars that are along its path. This affords astronomers a rare opportunity to study warping of space by Proxima’s gravity, as will be evident in the apparent displacement of the two stars in sky photographs. This effect is called gravitational lensing. The amount of warping will be used to calculate a precise mass for Proxima Centauri and look for the gravitational footprint and any planets orbiting the star. The background image shows a wider view of the region of sky in the southern constellation Centaurus that Proxima is traversing. Object Names: Proxima Centauri, Alpha Centauri C. 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)
Upcoming conjunction between Proxima Centauri and a background star
This diagram shows the upcoming conjunction between the nearest star to our Sun, Proxima Centauri, and a distant background star. Proxima’s gravitational field warps the space around it. This gravitational lensing effect distorts space like a funhouse mirror and bends the path of light from the background star. The result is that the apparent position of the star will shift slightly during Proxima Centauri’s passage, as seen in the upper right diagram. If an unseen planet is orbiting Proxima Centauri the star’s apparent position will be further offset, as seen at lower left. Credit: A. Feild (STScI)


Why is this research important to Astrobiology?

The search for small planets around distant stars is of major importance to NASA’s Astrobiology Program. Earth is our only example so far of a habitable planet where life is able to survive. Discovering planets of similar size to Earth is a step toward identifying habitable worlds beyond the Solar System.

Many extrasolar planets have been discovered, but most are gas giants similar to Jupiter. It’s much more difficult to spot small planets, but new technology and techniques are now making this possible. For some information on possible habitable worlds out amongst the stars, check ou the Habitable Exoplanets Catalog.


Where can you read more about this research?

A research paper concerning this research has been submitted to the Astrophysical Journal. Dr. Sahu recently presented the results at the 222nd meeting of the American Astronomical Society in Indianapolis, Indiana. The Science Program from the meeting is available at: http://aas.org/meetings/aas-222nd-meeting


An artist impression of a trip to Proxima Centauri. Credit: Mana Brau

Astrobiology Magazine contacted Kailash Sahu of the Space Telescope Science Institute, Baltimore, Maryland, with some additional questions about the research:

Astrobiology Magazine (AM): The press release mentions Hubble, Gaia and VLT. Are there any other instruments that will be used to make observations of this event?

Sahu: Some clarification first. We mentioned these telescope which we thought can be used for these observations. The idea of the paper is to alert the astronomical community about this possibility so that they can plan, request for telescope time, etc. We do have approved time with Hubble for a limited set of observations (to measure the mass of Proxima). But searching for planets needs lots of time and effort which HST alone will not be able to do, so efforts from other telescopes (such as VLT and GAIA) will be very useful. Some other facilities such as GEMINI may also be helpful.  

There is one part where even smaller, 1-meter class telescopes may be able to contribute. If Proxima has a planet that comes very close to the source star, the source star may brighten due to microlensing by the planet. Since the planet’s Einstein ring is small (about 8 milliarcsec for a Jupiter-mass planet), the probability of such photometric brightening is small. But the payoff would be high since the timescale of brightening provides a direct measure of the mass of the planet. Also, if Proxima has a planetary system, the probability of brightening by a planet increases.  Such brightening would last anywhere between a few hours to a few of days, and can happen anytime within a period of about 100 days. So such an event may be best observed by a network of smaller telescopes.

AM: In an ideal world, where would be the best place on Earth or in orbit for astronomers to view the event?

Sahu: Proxima is a southern (declination of –62 degrees) object, so it is best observed from the southern hemisphere. Fortunately, it is a circumpolar object in places like Chile, Australia  and South Africa, so it can be observed all through the year. Space based telescopes have some extra advantages of good image quality and no atmospheric scintillation which help the astrometric observations, but ground-based telescopes with adaptive optics can also be used for astrometry. 

AM: Previous observations have ruled out Neptune and Jupiter sized planets around Proxima Centauri. Is there any indication yet that smaller planets may be in orbit, or is this the first chance to make the type observation needed to determine this?

Sahu: A small correction: The radial velocity observations rule out planets with (m sin i) larger than Neptune mass up to 1 AU (where i is the inclination angle). So, if there is a Neptune mass planet with an orbit that is face-on or farther away, that won’t be detected by radial velocity measurements. This current method is sensitive, in principle, to detecting Earth-mass planets, but the probability of detection decreases with mass of the planet.

AM: Have there been similar conjunctions in the past where observations of other stars have been made?

Sahu: No such observations have been reported so far, although such conjunctions for some other stars have been predicted in the past.

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