Four New Exoplanets Found Orbiting Nearby Star

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An international team of astronomers have discovered four Earth-sized planets—or exoplanets—orbiting Tau Ceti, the nearest Sun-like star to our solar system.

Roughly 12 light years away, the planets have masses as low as 1.7 times Earth’s mass, making them among the smallest planets ever detected orbiting a star similar to our own. Two of the four exoplanets are located in the habitable zone of the star, and could potentially host liquid water on their surfaces.

The study analyzed data collected by the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory in Chile, combined with the High-Resolution Echelle Spectrometer (HIRES) at the W. M. Keck Observatory on Maunakea volcano.

“HIRES is one of only a few spectrometers in the world that have routinely delivered the level of radial velocity precision needed for this kind of work,” said the study’s co-author Dr. Steve Vogt, professor of astronomy and astrophysics at University of California, Santa Cruz. “And it is one of only two instruments in the world, the other being HARPS, that has been able to deliver this precision level for over a decade. It is a very unique facility in the exoplanet discovery field.”


The four planets were detected by observing wobbles in the movement of Tau Ceti. Known as the Doppler effect, the subtle wobbling is created by the gravity exerted by orbiting planets on their host star. Measuring this minuscule variation requires techniques sensitive enough to detect changes in movement as little as 30 centimeters per second.

“We are getting tantalizingly close to the 10 centimeters per second limit required for detecting Earth analogs,” said Dr. Fabo Feng from the University of Hertfordshire in the United Kingdom and lead author of the study. “Our detection of such weak wobbles is a milestone in the search for Earth analogs and the understanding of the Earth’s habitability through comparison with these analogs.”

The same international team also investigated Tau Ceti in 2013, when Dr. Mikko Tuomi led an effort using the star to develop data analysis techniques.


“We came up with an ingenious way of telling the difference between signals caused by planets and those caused by a star’s activity,” said Dr. Tuomi. “We realized that we could see how a star’s activity differed at different wavelengths, then used that information to separate this activity from signals of planets.”

“We have painstakingly improved the sensitivity of our techniques and could rule out two of the signals our team identified in 2013 as planets. But no matter how we look at the star, there seems to be at least four rocky planets orbiting it,” Dr. Tuomi added. “We are slowly learning to tell the difference between wobbles caused by planets and those caused by stellar active surface. This enabled us to essentially verify the existence of the two outer, potentially habitable, planets in the system.”

Tau Ceti closely resembles our Sun in size and brightness. Both stars host multi-planet systems, giving rise to the possibility that Tau Ceti could provide a habitable living environment for humans.


“If the outer two planets are found to be habitable, Tau Ceti could be an optimal target for interstellar colonization, as seen in science fiction,” said Dr. Feng.

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