Arizona scientists find ‘Goldilocks’ planet

The largest of the newly discovered planets is 2.1 times the size of the Earth, while the smallest planet is 1.5 times larger than Earth (Artist rendition by Ryan Molton)
The largest of the newly discovered planets is 2.1 times the size of the Earth, while the smallest planet is 1.5 times larger than Earth and close to the habitable zone. (Artist rendition by Ryan Molton)

An Arizona scientist is leading a team that has gotten one step closer to finding the ingredients for life in outer space.

Ian Crossfield, an astronomer at the University of Arizona, is leading a group of scientists from around the world who, last month, discovered a small planet that is in the right zone around its star where water could exist.

On Jan. 6, Erik Petigura, a graduate student at UC Berkeley, was examining data collected from the NASA Kepler Space Telescope Mission using software he designed to track changes in stellar brightness. Petigura discovered a red-dwarf star showing three different changes in its brightness. This led him to believe the three planets were closely orbiting the star, crossing in front of it during their orbit, and periodically blocking some of the starlight. After additional research, Crossfield proved that they had discovered a new planetary system.

The star, called EPIC 201367065, is not Earth’s closest neighbor, sitting more than 35 times farther away than our closest star but this M-Dwarf star is one of the closest stars known to have planets that cross the star from Earth’s vantage point.

The discovery is significant because the smallest of the three may be the closest, Earth-sized planet in the habitable zone known to date, Petigura said.

The Kepler Space Telescope, which orbits the Earth, was designed to survey the universe and find Earth-like exoplanets existing in the habitable zone of their planets. This zone, also known as the “Goldilocks zone,” is the area surrounding a star where conditions are just right so liquid water could exist on the surface of a planet. According to Petigura and Crossfield, the smallest planet, which is about 1.5 times the size of Earth and has an orbit of about 30 days, could be in this zone.

“We’re also hoping to observe the planets using the Hubble Space telescope to start measuring what their atmospheres might be made of,” Crossfield said. “It’s a second confirmation to see if their atmospheres are mostly made of hydrogen or other molecules to start figuring out what conditions might be like there.”

An artists rendition of the surface of the smallest planet. Because the outer planet is only 1.5 times the size of the Earth, it may have a similar, rocky density. (Artist rendition by Ryan Molton)
An artists rendition of the surface of the smallest planet. Because the outer planet is only 1.5 times the size of the Earth, it may have a similar, rocky density. (Artist rendition by Ryan Molton)

When a planet passes in front of its star, their light is blocked, leading to a drop in brightness. When this is observed at different wavelengths of light, astronomers can use a method known as spectroscopy to determine what molecules are present in the atmosphere. Water, for example, is best observed in the infrared spectrum of light, said Travis Barman, an associate professor at the UA who also was involved in the research.

While finding traces of water in the atmosphere of these new planets would be an interesting discovery, astronomers are wary with people assuming a planet in the habitable zone must contain life.

“It’s easy to get caught up in the word habitable zone and start talking about life, but we don’t know yet all the details that you would need to allow life to form and flourish,” Barman said. “Instead, we can start to learn about the diversity of the atmospheric properties of planets in the habitable zone.”

One factor that influences the location of the habitable zone, is the type of star the planets are orbiting, especially in regards to the brightness of the star, Barman said.

These three planets orbit an M-dwarf star, meaning the star is smaller and cooler than the sun. Because of this, the planets must be closer to the star to have a “pleasant” surface temperature, said Crossfield.

The properties of the planets’ atmospheres can also change the temperature on the surface of the planets, affecting the reach of the habitable zone. For example, Earth’s atmosphere creates a greenhouse effect that changes the temperature at its surface, so the distance from the sun doesn’t solely influence its overall temperature, Barman said.

“The Earth has a nice, regulating cycle that involves geologic activity and keeps the greenhouse gases at just the right level to keep our temperatures comfortable, and so a planet that doesn’t have geologic activity can affect how stable those temperatures can be,” Barman said. “Life is a very delicate thing so things tend to be just right.”

When the Kepler Space Telescope was launched in 2009, its main goal was to find and explore different planetary systems throughout the universe. To do this, the space-based telescope needed to look at a huge number of stars to track changes in brightness to see if the stars had planets orbiting them.

Initially, Kepler found more than 1,000 exoplanets of various sizes, some of which were in or near the habitable zones of their stars, according to NASA. While Kepler has found lots of planets, many of them are large and similar to Jupiter in size, since they are easier to find. Earth-sized planets have still been found, but many of them are not in the habitable zone or if they are, they are too far away for in-depth study. This find is significant because one of the planets is slightly larger than Earth, close to the habitable zone, and close enough for in-depth, atmospheric study, Crossfield said.

“[This discovery] is exciting for at least two reasons: First, because K2 is working and it is finding these systems around small M-dwarfs that we hoped to find so that means we can expect to find a lot more in the coming years,” Crossfield said. “It’s also exciting because with this particular system, the planets should be very good for follow up observations… to study their atmospheres to find out what they are made of.”

Michaela Kane is a reporter for the Arizona Sonora News, a service from the School of Journalism with the University of Arizona. Contact her at

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