Once NASA’s Nancy Grace Roman Space Telescope launches in the next 12 to 18 months, it will be on its way toward outdoing scientists’ initial expectations. Researchers have confirmed that Roman should be able to measure enormous seismic waves rippling across the surfaces of more than 300,000 red giant stars.
Roman is a survey telescope, with an 8-foot (2.4-meter) mirror like the Hubble Space Telescope, but a field of view 100 times larger. Besides studying dark matter and dark energy, one of Roman’s core surveys will be the Galactic Bulge Time-Domain Survey, in which millions of stars in the central bulge of the Milky Way galaxy will be studied, principally to look for exoplanets. The idea is to use gravitational microlensing as a planet-finding device. Gravitational lensing is a technique often used in astrophysics to study distant objects; due to the way spacetime warps as per general relativity, some huge objects in space (like galaxy clusters, for instance) warp light traveling nearby, therefore magnifying, distorting and duplicating the source of that light as seen through our telescopes. Gravitational microlensing refers to gravitational lensing on smaller scales, like that of a planet.
The science of studying these stellar oscillations is called asteroseismology, and the frequency of the oscillations can reveal the masses, sizes and ages of the stars for which they are observed. In turn, understanding stars better can inform astronomers as to some of the properties of the planets that orbit them.
“With asteroseismic data we’ll be able to get a lot of information about exoplanets’ host stars and that will give us a lot of insight on exoplanets themselves,” study leader Trevor Weiss of California State University, Long Beach, said in a statement.
The Kepler Space Telescope, which hunted for exoplanets by watching for transits, was able to make asteroseismological measurements of 150,000 stars. In assessing whether Roman will be able to do the same, Weiss’ team applied the Kepler dataset to models of Roman’s observational capabilities. In particular, they discovered that Roman will be adept at detecting stellar oscillations on red giant stars, which are both luminous (making them easier to detect) and have a high frequency of oscillation with a period ranging from hours to days. This is a good match for Roman’s Galactic Bulge Time-Domain Survey, which will keep a steady eye on hundreds of millions of stars in the Milky Way galaxy’s bulge every 12 minutes over half-a-dozen 70.5-day stretches, meaning that it will be attuned to the red giants’ vibrations.
“Asteroseismology with Roman is possible because we don’t need to ask the telescope to do anything it wasn’t already planning to do,” said Marc Pinsonneault of Ohio State University. “The strength of the Roman mission is remarkable: it’s designed in part to advance exoplanet science, but we’ll also get really rich data for other scientific areas that extend beyond its main focus.”
The bulge, which harbors the supermassive black hole Sagittarius A*, is the oldest part of the Milky Way galaxy. Many of its stars are now aging out, evolving off the main sequence (which is what we call the stage of their life when they are generating energy through the fusion of hydrogen into helium in the core).
Upon leaving the main sequence, the next stage in the evolution of a sun-like star with less than eight solar masses is to expand and become a red giant. Initial estimates of the number of red giants that Roman could observe seismic waves on was 290,000, but deeper analysis found that the actual number could be much more.
“Now that we know the survey will entail a 12-minute cadence, we find it strengthens our numbers to over 300,000 asteroseismic detections in total,” said Weiss. Depending upon certain assumptions, the total number could be as high as 648,000 red giants in its field of view, with 358,000 in the bulge.
“It would be the largest asteroseismic sample ever collected,” said Weiss.
Understanding the properties of the host stars will inform astronomers about the planets they find — for example, whether they are in the habitable zones. The observations will also provide clues as to the future of planetary systems when their star begins to gradually die by evolving into a red giant star, before casting off their outer layers and leaving behind a dead white dwarf. How soon this happens depends upon the star’s mass. More massive stars live shorter lifetimes than less massive stars. During the expansion and casting off phase, any planets orbiting close to the star are destroyed.
In our solar system‘s case, Mercury, Venus and probably Earth will all be doomed. However, microlensing has an advantage by being able to detect planets that are farther out from their star, far enough out to perhaps survive the red giant stage. By detecting planets around red giants, and the orbits of those planets, it will help astronomers better understand what fate will befall the planets of our solar system, and how far out a world has to be in order to survive. Astronomers have already noticed a deficit of planets orbiting red giants, and Roman’s findings will cement our picture of evolved planetary systems.
“Our work will lay out the statistical properties of the whole population — what their typical abundances and ages are — so that the exoplanet scientists can put the Roman measurements in context,” said Pinsonneault.
Roman’s asteroseismic discoveries won’t just teach us about planetary systems, but the ages of the stars based on the asteroseismic readings will act as a guide to the history of the Milky Way, and its bulge in particular.
“We actually don’t know a lot about our galaxy’s bulge since you can only see it in infrared light due to all the intervening dust,” said Pinsonneault. “There could be surprising populations or chemical patterns there. What if there are young stars buried there? Roman will open a completely different window into the stellar populations in the Milky Way’s center. I’m prepared to be surprised.”
For example, a young population of stars could come to light if Roman measures oscillations on more massive red giants. This is because more massive stars live shorter lifetimes and therefore would have formed more recently.
The Roman Space Telescope is currently scheduled for launch between autumn 2026 and May 2027. In the meantime, the new assessment of its asteroseismic capabilities has been published in The Astrophysical Journal.