Looking for Earth 2.0 in Binary Systems

While astronomers have found thousands of exoplanets over the last few decades, the true prize continues to elude them - they have yet to find an Earth-mass, rocky planet orbiting in the habitable zone of a Sun-like star. That’s partially just due to cosmic geography - around half of all sun-like stars near us aren’t alone. They have one or more companion stars that complicate their orbital dynamics, as well as those of any planets they might host. But a new paper, available in pre-print on arXiv, suggests a new NASA Small Explorer mission called Searching for Habitable Exoplanets with Relative Astrometry (SHERA), which aims to use those complex dynamics to help find Earth-sized worlds in these multi-star systems. Currently, there are three main ways that astronomers hunt for exoplanets, and while we’ve covered them both in great detail here at UT, it’s worth a recap to show how SHERA fits into the mix. Missions like Kepler and the Transiting Exoplanet Survey Satellite (TESS) use a technique called transiting - they watch for slight dips in a star’s light as a planet passes in front of them. While the technique has been wildly successful, it requires a significant amount of luck, as the orbit has to align with Earth in order for the technique to be most effective. A second method, practiced by many ground-based spectrographs, is known as radial velocity (RV) measurements. These measure the “wobble” in a star caused by the planet orbiting it. But an Earth-like planet tugging on a Sun-like star from one astronomical unit (AU) only moves its star around 9 cm/s - and the limit of our current suite of instrumentation is around 50 cm/s due to activity on the star’s surface - so this technique is out for finding an Earth analogue. Fraser discusses the TOLIMAN mission - a precursor to SHERA. That leaves one other option - astrometry. This technique measures the exact position of stars against a background field of, you guessed it, other stars. But again, astronomers have run into problems with sensitivity. For an Earth-twin orbiting around a Sun-like star 33 light years away, the “wobble” the planet would induce to show up as a movement of around 0.3 microarcseconds (uas). Our most powerful astrometry mission - GAIA - currently has a precision of around 100 uas, several orders of magnitude above what would be needed to find this type of planet. SHERA takes a page from GAIA’s book, but with a twist. Instead of mapping a star against the faint background of stars, which can add a baseline level of “noise” that it's hard to overcome, SHERA focuses on bright, but closely separated binary pairs of stars. Since binaries are tied together gravitationally, they act as co-moving reference points amongst the background. Precisely measuring the distance between them eliminates any background error introduced by background stars, and it also cancels out any large-scale optical distortions that might otherwise skew the data. But, if one of the Sun-like stars in the binary (and to be clear, they could both be Sun-like) has an exoplanet, that planet would cause a periodic shift in the relative separation. Which is exactly what SHERA is designed to detect. But doing so requires some very fancy technological wizardry akin to semiconductor manufacturing. Fraser talks about that Habitable Worlds Observatory (HWO) To get down to the required sub-microarcsecond precision level, SHERA uses something called a diffractive pupil. This is created by using an electron-beam lithography process (similar to that used to make computer chips) to imprint a complex phase pattern onto the mirror of a 22-cm space telescope. This complex pattern serves three main purposes. First, it acts like an optical ruler, measuring how optical distortions of the stars, as well as their thermal expansion, change over time. It also spreads starlight over multiple pixels, but in a known pattern, allowing the instrument to average away microscopic defects in the detector itself. And finally, it stops stellar activity from mimicking the signal expected by a planet by watching the star’s spectra and correcting for any changes in the spectrum and absorption lines that might be caused by a sunspot rather than a change in the relative distance between the two binaries. The SHERA team has come up with a list of 7 nearby binary systems within 17 parsecs of Earth. In doing so, it will attempt to achieve three primary science goals. First is to find any Earth-mass planets around the closest target stars. Second is to test whether close-in binaries suppress planet formation at temperature distances - we already know they do at some distances. Third is to offer additional context to ground-based radial velocity data, combining its astrometric data with RV measurements from the ground. According to the authors calculations, based on standard detection rates, the mission is expected to find around four small, habitable zone planets across its target stars. Importantly, even finding less than that would be important - they calculate that finding fewer than two planets in the sample would measure that single-star habitable zone planet occurrence rates around binaries are significantly lower than would be expected around single stars. But perhaps the most important thing about SHERA is another mission it will help facilitate - the Habitable Worlds Observatory (HWO). Thirteen of the fourteen stars in SHERA’s target list are classified as Tier 1 targets for HWO. Since that huge, expensive observatory could spend a significant amount of time just trying to prove an existence of a planet, SHERA, which is designed as a relatively inexpensive NASA Small Explorer (SMEX) mission, and is essentially a stepped up version of a TOLIMAN 16U CubeSat originally designed to target Alpha Centauri, could help lower the amount of time HWO takes to characterize planets around those stars (assuming there are any) by up to 40% - a massive time saving for a very expensive observatory. To be clear, there are no current plans to adopt SHERA as an official mission. HWO is planning to launch late next decade, or, perhaps more realistically, in the early 2040s, so there’s still a while to go before that use case would no longer be valid. Given its relatively small price tag, there’s a possibility the mission could even be privately funded - but for now at least, it’s unclear whether we’ll ever see this specialized planet hunter ever take to the skies. Learn More: J.L. Christiansen et al - Searching for Habitable Exoplanets with Relative Astrometry (SHERA). I. The Case for Searching for Planets in Binary Star Systems UT - A New Mission Will Search for Habitable Planets at Alpha Centauri UT - How NASA Plans to Build the Habitable Worlds Observatory UT - The Habitable Worlds Observatory Will Need Astrometry To Find Life