The Milky Way’s galactic bulge, that dense, bulbous region around the center of our galaxy, is basically the cosmic equivalent of a rush-hour subway car - stuffed with stars, planets, and the occasional free-floating object that got lost on its way home. For decades, astronomers have been peering at it with everything from ground-based scopes to NASA’s Hubble and James Webb space telescopes. But soon, NASA’s Nancy Grace Roman Space Telescope will be the first to make studying this jam-packed neighborhood a core part of its mission, promising to survey millions of stars and spot thousands of new exoplanets. Because apparently, the universe hasn’t shown us all its tricks yet.
To help Roman make sense of all that galactic clutter, astronomers used Hubble to observe many of the same areas of the bulge that Roman will target in its core Galactic Bulge Time-Domain Survey. By comparing Hubble data taken months or years earlier - because patience is a virtue, especially in astronomy - with Roman’s fresh snapshots, they hope to interpret Roman’s observations more accurately. Roman is slated to launch as soon as early September 2026, which gives everyone plenty of time to finish their crossword puzzles.
“A top priority of our Hubble survey is to cover as much sky area as possible,” said Sean Terry, project lead and assistant research scientist from the University of Maryland, College Park and NASA’s Goddard Space Flight Center. A paper detailing the team’s work was published on May 11, 2026, in the Astrophysical Journal, because apparently it’s not enough to just point telescopes at stuff - you have to write it down, too.
Many planetary systems in the Milky Way start out like our own solar system: a cosmic gas cloud collapses, a star grows up, and planets form around it. But in some systems, things go sideways - literally - and a planet gets ejected, becoming a “rogue planet” wandering the galaxy like a cosmic drifter. Roman’s Galactic Bulge Time-Domain Survey is expected to detect hundreds of these rogue planets, along with previously unseen isolated neutron stars and even black holes with masses similar to our Sun. Just your average celestial scavenger hunt.
The survey will consist of six 72-day observing seasons during which Roman will snap a picture every 12 minutes of a large chunk of the bulge - about 1.7 square degrees, 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. Microlensing events occur when the light from a distant object gets warped by the mass of a closer object along the line of sight, on a much smaller scale than galaxy-sized lensing events. Basically, it’s the universe’s way of playing hide-and-seek with exoplanets.
“The great thing about microlensing is that we’ll 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,” said co-author Jay Anderson of the Space Telescope Science Institute in Baltimore. Because when you’re taking a census, you don’t discriminate - even Martian-sized objects get counted.
When a telescope observes a lensing object, like a bright star, aligning with a star in the galactic bulge, it can be tricky to tell which star the light comes from. Timing is key: if astronomers can identify light sources separately before a microlensing event occurs, disentangling them becomes much easier. So, to collect pre-Roman data, astronomers used Hubble to conduct a large-scale survey starting in spring 2025, covering much of the same area Roman will observe. This survey is even larger than two previous projects (each around 0.5 square degrees) that produced Hubble’s largest mosaic - that of the Andromeda galaxy, which took over 10 years to assemble. Clearly, astronomers are not afraid of a little homework.
“The 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,” said Anderson. “When, in a couple of years, an event happens during Roman's long stare at the field, we can go back and say, ‘This was a red star, this was a blue star, and the event happened when the red star went in front of the blue star.’” Just like a cosmic traffic cam, but with better resolution.
Hubble’s data will also help analyze the lensing objects themselves. A microlensing event only measures the ratio of masses between a host star and its planet. But with data from stars before or after their microlensing events, scientists can measure the stars’ individual masses - similar to how Hubble previously determined the mass of a star and its planet in the Milky Way. This turns a fuzzy mass ratio into a much more precise measurement.
“Instead 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,” Terry said. Because sometimes you need to know not just that something is out there, but exactly how much it weighs.
While exoplanet discovery is a big part of Roman’s survey, observing such a large area with Hubble can also help identify areas of extinction - dense pockets of dust and gas that absorb or scatter light - allowing astronomers to create maps of where we can and can’t see stars. Hubble’s survey has also kick-started a brand-new catalog of stars, which will help characterize the host stars of exoplanets discovered by Roman. The research team predicts Roman will add to Hubble’s star catalog by an order of magnitude.
“This Hubble survey will build a catalog of 20 to 30 million point sources,” said Terry. “But, 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.” So, basically, Roman will be the overachiever in this cosmic family.
The data from the most recent Hubble survey is available in the Mikulski Archive for Space Telescopes - free of charge, no subscription required.
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA Goddard manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
The Nancy Grace Roman Space Telescope is managed at NASA Goddard, with participation by NASA's Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Space Telescope Science Institute; and a science team from various research institutions. The primary industrial partners are BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California. Because building a space telescope takes a village - or at least a few corporate giants.