What are the oldest objects in the universe? Ancient stars, exoplanets, and galaxies explained

Astronomy is like a time machine in that we can look at events that happened billions of years ago. By looking at objects from shortly after the Big Bang formed our universe 13.8 billion years ago, we can probe the “origin story” of cosmic things like galaxies, stars, and exoplanets.

Inverse talked to two astronomers at the Smithsonian Astrophysical Observatory to learn about the oldest things we can see or infer, and why studying such old things matters.

What are the oldest stars?

Smithsonian astronomer Warren Brown warns that we can only infer age when studying stars. As a single example of why this is hard, Brown tells Inverse: “The oldest stars should have very few metals in their atmospheres. The complicating part is that’s not always true.”

Astronomers use the terms “metal” or “metallicity” to describe the abundance of elements heavier than hydrogen or helium. Metal-poor stars tend to be old, such as “Methuselah” (HD 140283), which is at least 12 billion years old.

Hydrogen and helium, both lightweight elements, were abundant in the universe before the first stars formed. As stars fuse these elements in their cores, they generate heavier elements such as carbon, oxygen, and silicon. When the stars cease burning, often they will shed their gas gradually (as cooling white dwarfs) or explosively (as a supernova), distributing the heavier elements in the universe.

Brown explained that this process means that, generally, the oldest stars shouldn’t have any heavy elements in them because they were not available. But this is not always the case. As one example: “There’s fairly pristine gas in the interstellar medium,” Brown explained, referring to the gas between stars in mostly-empty space. Since few stars are available in these regions to generate older elements, only hydrogen and helium are present, for the most part.

Assuming that the star does not form in an unusual environment, the main way astronomers infer age is by looking at places with lots of old stars, such as globular cluster galaxies, Brown says.

Additionally, certain classes of stars tend to have characteristics of age associated with them. The most notorious are white dwarfs, the cooling remnant cores of stars like our sun that shed all their layers of gas. Measuring the temperature of the white dwarf provides a robust estimate of age, as that cooling happens at a predictable rate over time.

Brown added that often, astronomers require more information on a star’s spectrum before determining its age. The spectrum shows the abundance and proportion of elements visible in a star’s atmosphere.

An example is a recently discussed star in Nature, with an estimated age of 12.9 billion years, brought into view through a foreground galaxy bending the light through microlensing. Astronomers on that study are waiting on the newly launched James Webb Space Telescope to begin operations, as the telescope’s infrared light is specialized in looking at such far-away objects.

What are the oldest exoplanets?

Before discussing exoplanets, let’s briefly discuss redshift. Redshift is the phenomenon that occurs when an object’s light is shifted towards the red edge of the spectrum because it is moving away from us. Often, redshift happens due to the expansion of the universe; as objects recede from one another, their observable light gets stretched to red, or infrared.

That’s why Webb is so important, as it is optimized for infrared light and operates at a Lagrange point far from normal light interference from Earth. This allows it to gather as many light photons as possible using its large, hexagonal mirror that includes 18 segments spanning a width of 21 feet, 4 inches (6.5 meters).

Exoplanets are even trickier to look for than stars, given they are tiny and can only be seen through the reflected light from their parent star, or through their gravitational effects on that star. While we have more than 5,000 cataloged exoplanets, the population we know of is disproportionately made up of large planets closer to Earth. That’s simply because close-up planets are easier to view through the parent star's “wobbles,” or the reflected light.

A Science article from 2003 mentions a highly redshifted pulsar, B1620-26; a pulsar refers to a star that rotates swiftly and sends out a regular pattern of signals as it does so. A gas giant planet, roughly 2.5 times Jupiter’s mass, was discovered there in 2003. Scientists assume that the exoplanet is relatively old, given that the star system itself is approximately 12.2 billion years old. But as Brown explains, assumptions shouldn’t always be trusted.

“Exoplanet atmospheres have a real diversity of compositions, and it probably depends on the temperature and chemistry of how particles form at certain pressures and temperatures,” he explained. Complicating the matter is we cannot directly say, for example, what the planet is made of — absent making inferences based on its mass and radius, which often says (for example) if a planet is rocky or gaseous.

Astronomers are waiting for more advanced observatories that will render exoplanets in more detail besides points of light. Webb may be able to get information about closer and larger exoplanets, along with the forthcoming Extremely Large Telescope (ELT). But Earth-sized exoplanet information (along with our hopes for life) may have to wait a few decades for technology to improve.

What are the oldest galaxies and quasars?

Simply put, galaxies are collections of stars, and quasars are galaxies embedded with ultra-powerful supermassive black holes, generating immense amounts of X-rays visible from a great distance.

The furthest known candidate galaxy was just spotted this year and reported in The Astrophysical Journal. The candidate galaxy, HD1, appears to be 13.5 billion years old, but Smithsonian astronomer Fabio Pacucci warned that we need to gain more information on its spectrum before confirming its age. (That should come after Webb is operational, he said.)

HD1 may, coincidentally, also be the oldest quasar. Pacucci’s team suggests in the Monthly Notices of the Royal Astronomical Society that the galaxy is hosting a black hole of more than 100 million solar masses, which explains why the galaxy appears so bright in the data. (An alternative explanation is the galaxy is hosting a few very massive stars, which would also generate a lot of luminosity.)

The furthest confirmed quasar, Pacucci added, was discussed in 2021 in The Astrophysical Journal. At a redshift of 7.6, it has an age of roughly 13.1 billion years. But there’s a complication in our observations, he noted, as they don’t quite match the theory.

“You can't form a supermassive black hole right away,” he tells Inverse. The current model of black hole mergers suggests that they combine over time to create supermassive black holes, but the models suggest the creation happens over many billions of years. “The problem is that this process requires a very long time, and there is just not that much time after the Big Bang.”

Pacucci says the hope is Webb and observatories such as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile will provide more observational data that could be used to update the models.