The first stars in the universe had formed much later than what early observations of the cosmic microwave background (CMB) led scientists to believe, said the European Space Agency (ESA).

A new analysis was carried out using the agency's Planck satellite, which also revealed that the first stars were the lone sources necessary for reionizing atoms in the universe since they have already completed half of the process by the time the cosmos was about 700 million years old.

"The CMB can tell us when the epoch of reionization started and, in turn, when the first stars formed in the universe," said Jan Tauber, ESA Plank project scientist.

The world's current view of the universe features multitudes of galaxies and stars, so it can be a bit difficult to imagine what it was like when the 13.8-billion-year-old cosmos was just a mere seconds into its existence.

According to scientists, however, at that point, the universe was a hot and dense primordial soup containing mostly particles like neutrinos, protons, electrons and photons. It would also appear highly similar to an opaque fog, as photons are unable to travel long distances without colliding with electrons.

When the universe expanded, it cooled, becoming transparent after some 380,000 years had passed. Particle collisions had grown sporadic by then, with photons freely traveling across the universe.

A few hundred million years would pass before atoms start converging, eventually giving rise to the first stars in the universe. When these stars were born, their surrounding area was filled with light, which split atoms apart. Scientists call this the epoch of reionization. Soon, most of the universe was ionized completely, and it remained that way.

According to observations of the CMB, reionization was completed when the universe was about 900 million years old.

Planck scientists like Tauber used tiny fluctuations in the CMB polarization to see how the reionization process affected the universe and identify when the process began.

The first estimate of when the epoch of reionization began was provided by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) in 2003. According to it, the universe was just a couple hundred million years old when the first stars appeared.

However, further information gathered by the WMAP corrected the earlier estimate provided, moving the birth of the first stars to when the universe was between 300 and 400 million years old.

When the Planck Collaboration came into play, the birth of the first stars was moved yet again to when the universe was 550 million years old based on data from the first all-sky maps of the CMB polarization gathered using Planck's Low-Frequency Instrument.

Now, using Planck's High-Frequency Instrument, scientists have determined that the reionization process was a quick one. It started late, but the universe was half-reionized at about 700 million years.

Publishing their findings in two studies in the journal Astronomy and Astrophysics, Planck scientists suggest that the first stars can definitely be observed by astronomical facilities in the future and could even be within the observational reach of technology in place today.