Four Earth-sized planets detected orbiting the nearest sun-like star

This illustration compares the four planets detected
around the nearby star tau Ceti (top) and the inner planets of
our solar system (bottom). Credit: Fabo Feng

A new study by an international team of astronomers reveals
that four Earth-sized planets orbit the nearest sun-like
star, tau Ceti, which is about 12 light years away and
visible to the naked eye. These planets have masses as low as
1.7 Earth mass, making them among the smallest planets ever
detected around nearby sun-like stars. Two of them are
super-Earths located in the habitable zone of the star,
meaning they could support liquid surface water.

The were detected by observing the wobbles in
the movement of tau Ceti. This required techniques sensitive
enough to detect variations in the movement of the star as
small as 30 centimeters per second.

“We are now finally crossing a threshold where, through very
sophisticated modeling of large combined data sets from
multiple independent observers, we can disentangle the noise
due to stellar surface activity from the very tiny signals
generated by the gravitational tugs from Earth-sized orbiting
planets,” said coauthor Steven Vogt, professor of astronomy and
astrophysics at UC Santa Cruz.

According to lead author Fabo Feng of the University of
Hertfordshire, UK, the researchers are getting tantalizingly
close to the 10-centimeter-per-second limit required for
detecting Earth analogs. “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,” Feng said. “We have introduced new methods
to remove the noise in the data in order to reveal the weak
planetary signals.”

The outer two planets around tau Ceti are likely to be
candidate habitable worlds, although a massive debris disc
around the star probably reduces their habitability due to
intensive bombardment by asteroids and comets.

The same team also investigated tau Ceti four years ago in
2013, when coauthor Mikko Tuomi of the University of
Hertfordshire led an effort in developing data analysis
techniques and using the star as a benchmark case. “We came up
with an ingenious way of telling the difference between signals
caused by planets and those caused by star’s activity. We
realized that we could see how star’s activity differed at
different wavelengths and use that information to separate this
activity from signals of planets,” Tuomi said.

The researchers painstakingly improved the sensitivity of their
techniques and were able to rule out two of the signals the
team had identified in 2013 as planets. “But no matter how we
look at the star, there seem to be at least four orbiting it,” Tuomi said. “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.”

Sun-like stars are thought to be the best targets in the search
for habitable Earth-like planets due to their similarity to the
sun. Unlike more common smaller stars, such as the red dwarf
stars Proxima Centauri and Trappist-1, they are not so faint
that planets would be tidally locked, showing the same side to
the star at all times. Tau Ceti is very similar to the sun in
its size and brightness, and both host multi-planet systems.

A paper on the new findings was accepted for publication in the
Astrophysical Journal and is available online.

Explore further:

Astronomers discover ‘super-Earth’ planet orbiting nearby

More information: Color difference makes a difference:
four planet candidates around tau Ceti. arXiv.

Journal reference: arXiv

Provided by:
University of California – Santa Cruz