To be sure, this remains a tough measurement - in 2015 observations, position uncertainties were in the range of 10 milliarcseconds, a figure that needs to be reduced to hundred of microarcseconds.Ĭan ALMA handle this level of precision? Akeson’s team requested three pairs of observations in 2018-2019, with two achieved and the third incomplete, a dataset that will be complemented with future observations to identify deviations in the orbital motions of these stars. The relative positions of the two stars can be measured, as opposed to having to compare them to reference stars in other fields, meaning that the precision of the measurement is greatly increased. With ALMA, the proximity of Centauri A and B becomes an advantage. We’ve looked often in these pages at ALMA studies of protoplanetary disks (Akeson refers to the famous image of HL Tau, shown below, with the roughly one million year old disk dividing into clearly visible rings and gaps). At the longest baseline, the resolution is 12 milliarcseconds (one thousandth of an arcsecond, abbreviated mas). 5000 meters up on the Chajnantor Plateau in Chile’s Atacama desert, the site offers 66 antennas with baseline lengths of 150 meters to 16 kilometers. But for the nearest star system, Gaia is far less effective because of Alpha Centauri’s brightness.Įnter ALMA. Gaia should identify tens of thousands of exoplanets out to 1600 light years from the Sun, all of this by tracking minute changes in the stars’ position. Remember that figure from Jupiter as seen from 10 parsecs - the movement of the Sun is 500 µas. It’s a mission that has given astrometry a dazzling upgrade, offering 10 to 20 µas performance for a large sample of observed stars, making the upcoming release of its exoplanet catalog an event that should vastly enlarge our statistical understanding of planetary systems. This technique can also be used to identify planets around a star by measuring tiny changes in the star’s position as it wobbles around the center of mass of the planetary system. Image: Astrometry is the method that detects the motion of a star by making precise measurements of its position on the sky.
The beauty of astrometry is that it complements radial velocity by being best suited to planets with a wide separation from their star. This is something like the well established radial velocity technique for planet detection, but it operates within the plane of the sky instead of along the line of sight (radial velocity measures the Doppler shift in the star’s light as it is pulled alternately toward and then away from Earth along the line of sight). As the star’s position changes over time, the gravitational pull of an orbiting planet should likewise be revealed.
Keep that figure in mind.Īstrometry is about measuring the movement of a star’s position on the sky, and it has been put to good use for a long time in identifying binary star systems. She points out that if we viewed the Solar System from a distance of 10 parsecs, Jupiter’s impact on the movement of our star would be 500 microarcseconds (µas), which works out to 1.4 X 10 -7 degrees. Akeson’s case for using ALMA to make detections on the ground is robust, despite the challenges the method presents. ESA’s Gaia mission, launched in 2013, is likely to return a large horde of planets using astrometry as it creates a three-dimensional map of star movement in the Milky Way. Before that, numerous reported detections of planets around other stars, some going back to the 18th Century, have proven to be incorrect.īut we’re entering a new era. My interest is piqued by the fact that so few of the more than 4300 known exoplanets have been discovered using astrometry, although astronomers were able in 2002 to characterize the previously known Gliese 876 using the method. At the recent Breakthrough Discuss sessions, Rachel Akeson (Caltech/IPAC) made the case for using the technique with data from the Atacama Large Millimeter Array (ALMA). When it comes to finding planets around Centauri A and B, the method that most intrigues me is astrometry.
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