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The Xallarap Effect: Extending Gravitational Microlensing

‘Xallarap’ is parallax spelled backward (at least it’s not another acronym). And while I doubt the word will catch on in common parlance, the effect it stands for is going to be useful indeed for astronomers using the Nancy Grace Roman Space Telescope. This is WFIRST — the Wide Field Infrared Survey Telescope — under its new name, a fact I mention because I think this is the first time we’ve talked about the mission since the name change in 2020.

Image: High-resolution illustration of the Roman spacecraft against a starry background. Credit: NASA’s Goddard Space Flight Center.

While a large part of its primary mission will be devoted to dark energy and the growth of structure in the cosmos, a significant part of the effort will be directed toward gravitational microlensing, which should uncover thousands of exoplanets. This is where the xallarap effect comes in. It’s a way of drawing new data out of a microlensed event, so that while we can continue to observe planets around a nearer star as it aligns with a background star, we will also be able to find large planets and brown dwarfs orbiting the more distant stars themselves.

Let’s back up slightly. The gravitational microlensing we’ve become familiar with relies on a star crossing in front of a more distant one, a chance alignment that causes light from the farther object to bend, a result of the curvature of spacetime deduced by Einstein. The closer star acts as a lens, making light from the background star appear magnified, and the analysis of that magnified light can also show the signature of a planet orbiting the lensing star. It’s a method sensitive to planets as small as Mars.

Moreover, gravitational microlensing makes it possible to see planets in a wide range of orbits. While the alignment events are one-off affairs — the stars from our vantage point are not going to be doing this again — we do have the benefit of being able to detect analogs of most of the planets in our own system. What Shota Miyazaki (Osaka University) and colleagues have demonstrated in a new paper is that ‘hot Jupiters’ and brown dwarfs will also be detectable around the more distant star. Xallarap is their coinage.

David Bennett leads the gravitational microlensing group at NASA GSFC:

“It’s called the xallarap effect, which is parallax spelled backward. Parallax relies on motion of the observer – Earth moving around the Sun – to produce a change in the alignment between the distant source star, the closer lens star and the observer. Xallarap works the opposite way, modifying the alignment due to the motion of the source.”

Image: This animation demonstrates the xallarap effect. As a planet moves around its host star, it exerts a tiny gravitational tug that shifts the star’s position a bit. This can pull the distant star closer and farther from a perfect alignment. Since the nearer star acts as a natural lens, it’s like the distant star’s light will be pulled slightly in and out of focus by the orbiting planet. By picking out little shudders in the starlight, astronomers will be able to infer the presence of planets. Credit: NASA’s Goddard Space Flight Center.

The Roman telescope will be digging into the Milky Way’s central bulge in search of such objects as well as the thousands of exoplanets expected to be found through the older microlensing method, which Xallarap complements. And while microlensing works best at locating planets farther from their star than the orbit of Venus, xallarap appears to be better suited for massive worlds in tight orbits, which produce the biggest tug on the host star.

This will include the kind of ‘hot Jupiters’ we’ve found before but still have problems explaining in terms of their formation and possible migration, so finding them through the new method should add usefully to the dataset. In a similar way, the Roman instrument’s data on brown dwarfs found through xallarap should extend our knowledge of multiple star systems that include these objects, which are more massive than Jupiter but roughly the same radius.

Given that the view toward galactic center takes in stars that formed as much as 10 billion years ago, the Roman telescope will be extending the exoplanet search significantly. Until now, we’ve homed in on stars no more than a few thousand lights years out, the exception being those previously found through microlensing. The xallarap effect will complement the mission by helping us find older planets and brown dwarfs that fill in our knowledge of how such systems evolve. How long can a hot Jupiter maintain its tight orbit? How frequently will we find objects like these around ancient stars?

We should start getting useful data from both xallarap and more conventional microlensing some time in the mid-2020s, when the Roman instrument is due to launch. Nice work by Miyazaki and team.

The paper is Miyazaki et al., “Revealing Short-period Exoplanets and Brown Dwarfs in the Galactic Bulge Using the Microlensing Xallarap Effect with the Nancy Grace Roman Space Telescope,” Astronomical Journal Vol. 161, No 2 (25 January 2021). Abstract.


Comments on this entry are closed.

  • Whomever1 February 2, 2021, 13:29

    So, how do you pronounce xallarap? Also, will we get any information about the nearer object, other than its existence?

    • Paul Gilster February 2, 2021, 20:38

      Conventional microlensing still works, and xallarap (I don’t know how to pronounce it) is just an additional benefit from the same dataset. Standard microlensing will give us information about planets down to Mars size, though it’s not effective at distances below about that of Venus in our system. Microlensing is able to see planets far from the star, however, meaning gas giants in orbits similar to Jupiter and Saturn and farther. This is a realm that’s hard to explore with the other planet detection methods we commonly use.

      • Dominic Benford March 2, 2021, 13:08

        As I understand it, there is the possibility of measuring an exoplanet around the foreground star (and the microlensing statistics imply mostly finding smaller planets far from the star) as well as the background star (where the xallarap technique can most effectively find big, close-in planets). The signals are different enough that it ought to be straightforward to separate them. It might not be surprising if most of the xallarap events also have signs of exoplanets around the foreground star.

        From what I’ve heard, people seem to pronounce ‘xallarap’ with a Z at the beginning.

  • Michael February 2, 2021, 13:34

    Once we get our craft out to a couple of thousand AU’s and stop or slow them there we can use close stars to micro lens for us. For example if we could stop a probe at about 2 to 3 thousand AU’s and align it with Sirus we could use it as a micro lens say to view Gaia 1 a hidden star cluster 15 000 light years away.

    • torque_xtr February 3, 2021, 8:39

      It could be done much closer than that, and with much better results. If just the Gaia dataset contains a billion of stars, then their average projected density is about one star per 500 square arcseconds. If a close star with high proper motion and parallax of 0.1 arcseconds passes within 1 as of a distant star, as seen from Earth, than the line of perfect alignment passes within 10 AU from us, and from there the distant system can be observed almost as good as with FOCAL graviscope. I don’t remember the exact numbers, but calculated some time ago that within Jupiter orbit this happens up to several times per year for Gaia dataset only and for lenses with high proper motion. Much more often and close if slower-PM-stars are considered too. If someone finds a suitable event and sends a string of detectors into the caustic, this can be used as a good FOCAL demo :-)

  • Michael February 3, 2021, 14:03

    As a footnote, I remember asking them to add the Gaia dataset to Aladin back in 2016/2017, as you can see there are many opportunities of lensing events if we move out a little from our current plane of sight.

  • Alex Tolley February 3, 2021, 17:48

    Is this all possible because the telescope has much greater precision in measuring light intensity than existing telescopes? Reading the earlier Arxiv paper I see no allowance for measurement error, which I interpret as insignificant compared to the signal strength. Is that correct, or has this been glossed over?

    • Dominic Benford March 2, 2021, 13:15

      The power of Roman for this kind of work isn’t really the precision since the signals are fairly strong compared to the tiny changes seen in transit photometry (e.g. Kepler, TESS). Roman has high sensitivity and sharp images due to its space-based platform, and can see further into the galaxy using its infrared camera. Hence it monitors ~200M stars with high sensitivity and rapid cadence, which is about 1000 times as many as TESS can follow. Photometric uncertainties are definitely an issue but the dataset will be extraordinarily rich for internal calibration, and this should allow measurement close to the statistical limit.

  • Michael February 5, 2021, 8:05

    I hate to bang on about gravity lens but if you look at Sirius A and B the white dwarf is capable of bending astonishing amounts of power down the focal line from Sirius A ! If we take 1 sun radii around the white dwarf as an example, and we can go a lot bigger, and calculate the power it is around 8 to the power 20 watts at nearest A, B approach ! Aliens would love to have that power as would we. Stellar light sabres, these would also be common in Globular clusters.

    • Michael February 5, 2021, 15:13

      Could this beam be responsible for the reddening of Sirius potentially seen in the past, it has the power to certainly disrupt a comet or asteroid into dust to causing the reddening.

      • Andrei February 7, 2021, 10:42

        I am aware of that historical oddity of Sirius described as red. And you might be onto something here, since nothing in the normal evolution for Sirius A could have made it red and especially not in such a recent past. Yet there’s only one authority, Ptolemaios who said so. While other sources such a Romans and Chinese said white or ‘sea blue’.
        It seem more likely to me that people have swapped the dog star with Betelgeuse, even today with so much information easy at hand the misconception exist that the north star is supposed to be one of the brightest stars in the sky.

    • Mike Serfas February 5, 2021, 17:16

      Wow… now that’s an idea I’d like to see explored further. Not just as a random hazard for space pirates to watch out for … question: does this open the possibility of some sort of “habitable zone” around Sirius B in which planets receive abundant sunlight lensed from Sirius A during a portion of each year?

      • Michael February 6, 2021, 1:18

        At closest approach and with a Sirius B diameter of 12 000 km the focus starts to lands around 10 to 20 million km away. And if you take 1 m of light bent at the circumference and it focuses on a 1 square meter surface it would have around a 20 giga watt power luminosity ! If a 1 kg mirror was placed there it would undergo an acceleration of around 130 m per second per second and it gets bigger as it moves along the focul line. At around 500 AU’s the power rises to around 2200 giga watts at a meter squared and the same mirror would be accelerated towards 15500 m per second per second and the effect gets bigger further down the focul line before it diminishes. A comet would not survive a luminosity like that without a serous weight loss, it’s beyond supernova territory !

      • Michael February 6, 2021, 3:44

        Once you look at the wavefront combination the intensity in the middle of the beam is astronomical especially for the wavelength of Sirius A, Sirius B is a death star par excellance ! A way to use the power of this beam would most likely entail letting most of the power go through a centre hole, just to dangerous to use I would think.


  • Michael February 6, 2021, 18:39

    I forgot this is not a collimated beam, it should have undergone a square rule i.e. double the distance and quarter the brightness. So at closest approach of 8 AU’s the light will dim to a quarter with another 8 AU’s and so on. At 500 AU’s the light from Sirius A would only deliver around 600 megawatts per meter squared with 1 radii/m and the rest would have leaked through the focal line. It’s still a lot of power at that distance the way I understand it and still a huge amount of power flows down this corridor. A sail moving down this corridor would be accelerated to a very high velocity. I am unsure how the wavefront would change the dispersion pattern though, it may help or not, it may be worth modeling.

  • ljk May 24, 2021, 12:29

    It Could be Possible to see Gravitational Wave Lenses


    Gravitational-wave astronomy is very different from that of electromagnetic light. While gravitational waves are faint and difficult to detect, they also pass through matter with little effect. In essence, the material universe is transparent to gravitational waves. This makes gravitational wave astronomy a powerful tool when studying the universe. But it’s still in the early stages, and there is much to learn about how gravitational waves behave.

    Full article here:


  • Fredrick Germann June 15, 2021, 13:17

    The gravitational microlensing effort to find planets may be missing a trick. Even though splitting unresolved double stars in the Large Magellanic Cloud may not be a prime consideration for many planet hunters, doing so with the microlensing effort should bring more credit upon the effort and so benefit planet hunting as a side effect. What is needed is measuring a spectrum or at least a color for each detected microlensing incident and then measuring a corresponding spectrum or color after the microlensing expires. If this shows that only one of two portions of the spectrum, the portion from only one star, was brightened by microlensing, the pair of stars is now resolved. This may happen for only 1 in 100,000 incidents but planet hunters should be used to looking for rare events. Is this already done? Why do I not read about it?