They have dated the star, named Kepler-444, to 11.2 billion years old, making it the oldest star hosting Earth-sized planets found to date.

Astronomer Daniel Huber said it was extraordinary that such an ancient system of terrestrial-sized planets formed when the universe was just starting out.

"Kepler-444 is 2½ times older than our solar system, which is only a youthful 4.5 billion years old," said Dr Huber, a team member from the University of Sydney.

The discovery offers astronomers clues to their biggest unknown question - does life exist beyond Earth?

"In order to understand this question we need to know how and when planets like our Earth started forming, and how abundant are they," he said.

Kepler-444 is about 25 per cent smaller than the sun and 117 light-years from Earth. The planets circling the star vary in size (in a range between the size of planets Mercury and Venus) and have tight orbits, circumnavigating the star in just 10 days.

"Their closeness to their host star means they are uninhabitable because of the lack of liquid water and high levels of radiation," said Dr Huber.

However, the research provides important insights into whether another planet truly comparable to Earth may exist.

The international team, led by Tiago Campante of the University of Birmingham, used NASA's Kepler spacecraft.

To understand the star's properties, such as its diameter, mass and age, they employed a technique known as asteroseismology.

"It's a similar concept to using seismology here on Earth," Dr Huber said.

As sounds waves travel through a star, expanding and compressing gas, they cause tiny changes to the star's brightness, which instruments on Kepler can detect.

"By measuring the frequencies of sound waves from the brightness changes we can infer what the structure of the star is."

As a star ages the hydrogen gas inside its core fuses into helium. This process changes the star's core density, which affects the frequency of the sound wave travelling through it.

"We see these changes in the frequency patterns and can show, without a doubt, that this system of planets is 11 billion years old."

The size of the planets was inferred by the dimming that happens when they cross in front of the star.

The team's findings have been reported in The Astrophysical Journal.