Hubble Survey Sets Up Roman’s Future Look Near Milky Way’s Center

An observation (labeled “VISTA V V V Survey, Near-infrared) of the Milky Way’s center pointing toward its supermassive black hole, which is labeled Sagittarius A* (pronounced “A star”). At 7 o’clock from center is a small region, outlined with 5 fields of view from the Nancy Grace Roman Space Telescope that are stacked together in a horizontal rectangle. This region is about a third of the image in length and a fifth of the image in width. Within that same region are diagonal lines that alternate between orange and blue. A legend at the bottom left shows two Hubble instruments. One of them, marked with an orange diamond, is labeled “W F C 3 / U V I S.” The other, marked with a blue diamond, is labeled “A C S / W F C.” The background is a field of stars, gas, and dust that appear grey, except the center third that runs from the left to the right of the frame, which is composed of brown filaments of dust and gas. The tiny region surrounding Sagittarius A star appears orange. — This near-infrared image from the ground-based VISTA VVV Survey shows the galactic bulge near Sagittarius A* (pronounced “A star”), the black hole at the Milky Way’s center. The region, outlined in white, shows five stacked fields of view from NASA’s Nancy Grace Roman Space Telescope that will be observed as part of its Galactic Bulge Time-Domain Survey, one of its three core community surveys. Credits: Image: NASA, Alyssa Pagan (STScI)

This large-scale program provides a springboard to help interpret future Roman data.

One of the core community surveys of NASA’s Nancy Grace Roman Space Telescope, the Galactic Bulge Time-Domain Survey, is expected to locate over a thousand exoplanets that orbit far away from their stars, beyond the orbital distance of Earth from the Sun. Although Roman hasn’t launched yet, astronomers already are gathering useful supporting data by utilizing NASA’s Hubble Space Telescope, which could assist astronomers in analyzing Roman data.

The Milky Way’s galactic bulge, 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’s Hubble and James Webb space telescopes. Soon, NASA’s Nancy Grace Roman Space Telescope 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’s 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.

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. By comparing Hubble data taken months or years earlier to new Roman data, astronomers will be better able to interpret Roman’s forthcoming observations. The Roman telescope team is targeting as soon as early September 2026 for launch.

“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 in Greenbelt.

A paper about the team’s work published May 11, 2026 in the Astrophysical Journal.

‘Small’ lenses, large discoveries

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 “rogue planets” will be detected by Roman’s Galactic Bulge Time-Domain Survey, in addition to previously unseen, isolated neutron stars, and even black holes with masses similar to our Sun.

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.

Microlensing events, 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.

“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.

For Roman, from Hubble

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.

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’s largest mosaic, that of our neighboring Andromeda galaxy, which took over 10 years to assemble.

“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.’”

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’ individual masses, echoing the way Hubble previously determined 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.

“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. “So with the help of precursor imaging from Hubble you can hope to get direct measurements of the masses as opposed to indirect mass ratios.”