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This VISTA VVV Survey image shows the galactic bulge near Sagittarius A*, the supermassive black hole at the Milky Way\u2019s center. A region planned for observation by NASA\u2019s Nancy Grace Roman Space Telescope is outlined. This area has been observed by NASA\u2019s Hubble Space Telescope.<\/p>\n
Image: NASA, Alyssa Pagan (STScI); Acknowledgment: VISTA, Dante Minniti (UNAB), Ignacio Toledo (ALMA), Martin Kornmesser (ESO)<\/p>\n
The Milky Way\u2019s galactic bulge<\/a>, the bulbous region that surrounds the galactic center, contains a dense collection of stars, planets, and other free-floating objects. This region has been studied for decades with numerous ground-based and space-based telescopes, including NASA\u2019s Hubble <\/a>and James Webb <\/a>space telescopes. Soon, NASA\u2019s Nancy Grace Roman Space Telescope<\/a> will be the first to make studying the galactic bulge a part of its core science objectives, building on the data collected from all observatories before it. Roman\u2019s field of view will cover more area at a far faster cadence than previous space telescopes, allowing it to survey millions of stars and find thousands of new exoplanets.<\/p>\n To support Roman in characterizing numerous stars and planets, astronomers sought to use Hubble to observe many of the same areas of the galactic bulge that Roman will observe in its core Galactic Bulge Time-Domain Survey<\/a>. By comparing Hubble data taken months or years earlier to new Roman data, astronomers will be better able to interpret Roman\u2019s forthcoming observations. The Roman telescope team is targeting as soon as early September 2026 for launch.<\/p>\n \u201cA top priority of our Hubble survey is to cover as much sky area as possible,\u201d said Sean Terry, project lead and assistant research scientist from the University of Maryland, College Park and NASA\u2019s Goddard Space Flight Center in Greenbelt.<\/p>\n A paper about the team\u2019s work published May 11, 2026 in the Astrophysical Journal<\/a>.<\/p>\n Many planetary systems within the Milky Way evolve much like our solar system did, beginning with the collapse of a cosmic gas cloud, the growth of a star, and the formation of surrounding planets. However, in some systems, different events can result in a planet being ejected from the system where it formed. Hundreds of these \u201crogue planets\u201d will be detected by Roman\u2019s Galactic Bulge Time-Domain Survey, in addition to previously unseen, isolated neutron stars<\/a>, and even black holes with masses similar to our Sun.<\/p>\n This survey consists of six 72-day observing seasons during which Roman will take a snapshot every 12 minutes of a large portion of the bulge (approximately 1.7 square degrees of the region, or the area of 8.5 full moons). While it will detect a variety of targets, the survey is optimized to look for a specific type of event known as microlensing.<\/p>\n Microlensing events<\/a>, a type of gravitational lensing event, occur when the light from a more distant object is warped by the mass of a closer object along the line of sight. These events occur on a much smaller scale than larger lensing events (on the order of individual stars instead of galaxies or galaxy clusters) and allow us to search for exoplanets between us and the densely packed stars within the galactic bulge.<\/p>\n \u201cThe great thing about microlensing is that we\u2019ll be able to do a complete census of objects as small as Mars that are moving between us and these fields in the bulge, no matter what it is,\u201d said co-author Jay Anderson of the Space Telescope Science Institute in Baltimore.<\/p>\n When a telescope observes a lensing object, such as a bright star, aligning with a star in the galactic bulge, it can be difficult for astronomers to decipher which of the two the starlight comes from. Therefore, timing is a key consideration. If astronomers can identify light sources separately before a microlensing event occurs, it becomes far easier to disentangle them.<\/p>\n To collect this pre-Roman data, astronomers used the Hubble Space Telescope to conduct a large-scale survey, which began in the spring of 2025, covering much of the same area that Roman will observe in the Galactic Bulge Time-Domain Survey. The size of this program is even larger than two previous surveys (each around 0.5 square degrees) that led to Hubble\u2019s largest mosaic<\/a>, that of our neighboring Andromeda galaxy<\/a>, which took over 10 years to assemble.<\/p>\n \u201cThe main goal of these observations is to be able to identify objects that participate in lensing events during the Roman survey, catching them before they undergo the lensing event,\u201d said Anderson. \u201cWhen, in a couple of years, an event happens during Roman’s long stare at the field, we can go back and say, \u2018This was a red star, this was a blue star, and the event happened when the red star went in front of the blue star.\u2019\u201d<\/p>\n The data from Hubble also will help shape the analysis of the lensing objects themselves. The microlensing event itself measures only a ratio of the masses of a host star and its planet. With data from stars before or after their microlensing events, however, scientists would be able to measure the stars\u2019 individual masses, echoing the way Hubble previously determined<\/a> the mass of a star and its planet in the Milky Way. This method turns a more opaque measurement of the relationship between a star and its planet into one far more certain.\u00a0<\/p>\n \u201cInstead of estimating a mass ratio of a planet that’s orbiting a star, we can say that we’re confident it’s a Saturn-mass planet orbiting a star that’s 0.8 solar masses, for example,\u201d Terry said. \u201cSo with the help of precursor imaging from Hubble you can hope to get direct measurements of the masses as opposed to indirect mass ratios.\u201d<\/p>\n While exoplanet discovery is a large part of Roman\u2019s Galactic Bulge Time-Domain Survey, observing such a large area with Hubble also can help identify areas of extinction, dense pockets of dust and gas that absorb or scatter light, allowing us to create maps detailing where we can see stars and where we can\u2019t.<\/p>\n Hubble\u2019s survey also has provided the crucial beginning of a brand-new catalog of stars, which will help astronomers characterize the host stars of exoplanets discovered by Roman. The research team predicts Roman will add to Hubble\u2019s star catalog by an order of magnitude.<\/p>\n \u201cThis Hubble survey will build a catalog of 20 to 30 million point sources,\u201d said Terry. \u201cBut, by the end of the Galactic Bulge Time-Domain Survey, Roman may measure about 200 to 300 million, and it will produce, essentially, some of the deepest images ever taken of any part of the sky.\u201d<\/p>\n