A new exoplanet is the most promising candidate for a second Earth — where a year lasts 19.5 days

• NASA’s algorithm misidentified a planet that was discovered in 2018 but wasn’t flagged as one, which could support human life.
  
• In a new study published in Astrophysical Journal Letters scientists believe that Kepler 1649c has the potential to host liquid water and be the around the right temperature to support life.
  
• There is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system other than Kepler 1649c.
  
• However, since it’s a lot closer to its star than Earth is to the Sun, a year only lasts around 19.5 days.
  
• Since the planet was misidentified, its reclassification allowed scientists to look at its entire home system in a new way hinting at other planets that may also be potential candidates for a second-Earth.
  

NASA was looking at old Kepler data from 2018 when they realised one of the planets, Kepler 1649c, had been misidentified. The exoplanet is 300 lightyears away and only 1.06 times larger than our own planet.

Its rocky surface is capable of supporting liquid water and receives 75% of the light that the Earth receives from the Sun. According to NASA, this means the temperature on the exoplanet’s surface could also be similar to Earth.

The only difference is that Kepler 1649c’s star is a red dwarf and the Sun is a yellow dwarf.

"This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington.

The distance between Kepler 1649c and its star is around the same as Venus and the Sun. According to NASA’s calculations, this could mean that a year on the exoplanet only lasts around 19.5 Earth days.

The best hope for a second Earth so far
There are other exoplanets estimated to be closer to Earth in size, such as TRAPPIST-1f and, by some calculations, Teegarden c. Others may be closer to Earth in temperature, such as TRAPPIST-1d and TOI 700d.

However, there is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system other than Kepler 1649c.

"Out of all the mislabeled planets we've recovered, this one's particularly exciting – not just because it's in the habitable zone and Earth-size, but because of how it might interact with this neighboring planet," said Andrew Vanderburg, lead author on the paper released today in The Astrophysical Journal Letters.

Are there other options?
Since the planet was misidentified, its reclassification allowed scientists to look at its entire home system in a new way.

In the past, for every nine times the outer planet in the system orbits the host star, the inner planet orbits almost exactly four times. The fact that their orbits match up in such a stable ratio indicates the system itself is extremely stable and likely to survive for a long time.

More importantly, such near-perfect period ratios are often caused by a phenomenon called orbital resonance. Even so, a nine-to-four ratio is relatively unique among planetary systems.

Though unconfirmed, the rarity of this ratio could indicate the presence of a middle planet with which both the inner and outer planets revolve in synchronicity, creating a pair of three-to-two resonances.

The team looked for evidence of such a mystery third planet, with no results. However, that could be because the planet is too small to see or at an orbital tilt that makes it impossible to find using Kepler's transit method.

"The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-size exoplanets are common around these kinds of stars," said Vanderburg.