Washington, May 23 (IANS): Using NASA's Kepler space telescope, scientists have confirmed that the outermost and least understood planet in the TRAPPIST-1 system orbits its star every 19 days.

Scientists announced that the TRAPPIST-1 system has seven Earth-sized planets at a NASA press conference on February 22.

NASA's Spitzer space telescope, the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) in Chile and other ground-based telescopes were used to detect and characterise the planets.

But the collaboration only had an estimate for the period of the outermost planet --TRAPPIST-1h.

In a study published in the journal Nature Astronomy, researchers used data from the Kepler spacecraft to confirm that TRAPPIST-1h orbits its star every 19 days.

"It really pleased me that TRAPPIST-1h was exactly where our team predicted it to be. It had me worried for a while that we were seeing what we wanted to see -- after all, things are almost never exactly what you expect them to be in this field," said lead author of the study Rodrigo Luger, doctoral student at University of Washington in Seattle.

"Nature usually surprises us at every turn, but, in this case, theory and observation matched perfectly," Luger said.

TRAPPIST-1 is only eight percent the mass of our Sun, making it a cooler and less luminous star.

It is home to seven Earth-size planets, three of which orbit in their star's habitable zone -- the range of distances from a star where liquid water could pool on the surface of a rocky planet.

The system is located about 40 light-years away in the constellation of Aquarius and is estimated to be between three billion and eight billion years old.

At six million miles from its cool dwarf star, TRAPPIST-1h is located beyond the outer edge of the habitable zone, and is likely too cold for life as we know it.

The amount of energy (per unit area) planet h receives from its star is comparable to what the dwarf planet Ceres, located in the asteroid belt between Mars and Jupiter, gets from our Sun.

"It's incredibly exciting that we're learning more about this planetary system elsewhere, especially about planet h, which we barely had information on until now," Thomas Zurbuchen, Associate Administrator of NASA's Science Mission Directorate at Headquarters in Washington, said.

"This finding is a great example of how the scientific community is unleashing the power of complementary data from our different missions to make such fascinating discoveries," Zurbuchen said.