The field of exoplanet studies has grown by leaps and bounds in the past twenty years. To date, over 5,900 planets have been confirmed in more than 4,400 planetary systems. Astronomers have even confirmed the presence of a multi-planet system around Proxima Centauri, the closest star outside the Solar System. And yet, astronomers have not confirmed the presence of any exoplanets around Alpha Centauri, the binary system located about 4.344 light-years from Earth (which forms a trinary with Proxima Centauri).
Nevertheless, efforts to determine whether Alpha Centauri has any planets are ongoing, especially regarding the system's primary, the Sun-like star Alpha Centauri A (Alpha Cen A). Thanks to sophisticated infrared imaging of the James Webb Space Telescope (JWST), astronomers may finally be able to detect them. In a recent paper, an international team of researchers announced the preliminary findings of their observation campaign. According to their analysis, the JWST could detect a Jupiter-sized planet and a bright zodiacal dust disk around Alpha Cen A.
The research was led by Aniket Sanghi, a graduate student and NSF Graduate Research Fellow at the Cahill Center for Astronomy and Astrophysics at the California Institute of Technology (Caltech). He was joined researchers from the NASA Exoplanet Science Institute (NExScI), the French-Chilean Laboratory for Astronomy, the Laboratory for Instrumentation and Research in Astrophysics (LIRA), the Steward Observatory, the National Radio Astronomy Observatory (NRAO), the Max-Planck-Institute for Astronomy (MPIA), the Space Telescope Science Institute (STScI), NASA's Ames Research Center, and NASA's Jet Propulsion Laboratory.
The study of exoplanets has benefited from many advances in astronomy. These include adaptive optics (AO), coronographs, spectrometers, and machine learning algorithms. Nevertheless, observing exoplanets in binary star systems remains notoriously difficult, even with next-generation telescopes like the JWST. As he explained to Universe Today via email:
"The challenge arises because Alpha Cen A is in a close binary system with companion star Alpha Cen B. The presence of a second star can disrupt both the formation and evolution of planets in the system through gravitational interactions. That said, numerical simulations suggest that stable, long-lived planets can exist around Alpha Cen A. But for direct imaging efforts, there's another hurdle—age. Since Alpha Cen A is about 5 billion years old, any stable planets would have cooled significantly after losing their initial heat from formation. This makes them quite faint and difficult to detect."
Despite its advanced optics and extreme sensitivity to infrared radiation, direct imaging studies of Alpha Centauri (despite its proximity) presented a major challenge to Sanghi and his colleagues. While Webb's coronographs can block out light from one star, the companion is still capable of polluting observations. To block Alpha Cen A's starlight, the first step is to precisely position Alpha Cen A behind the coronagraph. This cannot be done with the standard JWST target acquisition process because of Alpha Cen A's tremendous brightness.
This meant that Sanghi and his team had to rely on a blind offset, a technique in which a nearby brighter reference star is used with pre-calculated offsets to observe an object. In this case, the reference star was Epsilon Muscae, a red giant star located about 330 light-years from Earth in the constellation Musca with a luminosity of 1,700 times that of the Sun.
"One of the major difficulties comes from the fact that Alpha Cen A is part of a binary system," said Sanghi. "Even though we can use the coronagraph to block out the overwhelming glare from Alpha Cen A, its companion star, Alpha Cen B, remains unobstructed, flooding the detector with light. This extra brightness complicates our efforts to cleanly subtract starlight contamination and confidently identify any faint signals from planets or dust disks."
Their results established an upper limit on the presence of a Jupiter-sized planet orbiting α Cen A at a distance of 1.5 to 2 AU - one and a half to two times the distance between the Sun and Earth. As Sanghi explained, this is just the beginning of Webb's observation campaign:
"Our early findings are a peek into just how powerful JWST is–we now know it is capable of detecting Jupiter-sized planets around Alpha Cen A in areas of the image where starlight contamination is low and where the coronagraph allows the most light from a potential planet to pass through. We’re still working to understand whether a potential planet might have slipped past us in less favorable spots. So while we didn’t find a Jupiter yet, the hunt is far from over!"
Their observations also accounted for a zodiacal disk, a thick circumsolar cloud of dust particles produced by asteroid collisions and cometary activity. In the Solar System, this interplanetary dust ring is distributed along the Sun's ecliptic (the plane where the planets orbit). This dust is responsible for what is known as "zodiacal light," a diffuse glow that can be seen along the solar ecliptic before sunrise or after sunset. According to their observations, this disk is roughly five times as bright as the Solar System's zodiacal disk.
"This research note presents just a first look at one of three JWST observations of Alpha Centauri A," said Sanghi. "In upcoming papers, we'll share results from all three visits and detail the sophisticated analysis techniques we're using to tackle this complex dataset. These JWST observations offer the tantalizing opportunity to search for planets and dust in the nearest stellar system to our own–there's a lot more to come."
This study, which recently appeared in the Research Notes of the American Astronomical Society (RNAAS), is part of a series of papers that provides a complete analysis of all JWST/MIRI observations of Alpha Centauri A. The research is part of Sanghi's doctoral work at Caltech's Exoplanet Technology Laboratory (ETLab), where co-author Dimitri Mawet (the David Morrisroe Professor of Astronomy and a Senior Research Scientist at NASA JPL) is his advisor.
Further Reading: RNAAS
