This
illustration compares the four planets detected around the nearby star
Tau Ceti (top) and the inner planets of our solar system (bottom). Credit: F. FENG, University of Hertfordshire, United Kingdom
Maunakea, Hawaii – A new study by an
international team of astronomers reveals that Tau Ceti, the nearest Sun-like
star about 12 light years away from the Sun, has four Earth-sized planets
orbiting it.
These planets have masses as low as 1.7 Earth mass, making them among the smallest planets ever detected around the nearest Sun-like stars. Two of them are Super-Earths located in the habitable zone of the star and thus could support liquid surface water.
The data were obtained by using 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, Hawaii.
“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 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 the wobbles in the movement of Tau Ceti. This wobble, known as the
Doppler effect, happens when a planet’s gravity slightly tugs at its host star
as it orbits.
Measuring Tau Ceti’s wobbles required techniques sensitive enough to detect variations in its movement as small as 30 centimeters per second. The smaller the planet, the weaker its gravitational pull on its host star, and the harder it is to detect the star’s wobble.
"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 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 Dr. Mikko Tuomi led an effort in developing
data analysis techniques and used 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 a star's activity. 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," said Dr. Tuomi.
"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."
Sun-like stars are thought to be the
best targets for searching for habitable Earth-sized 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 they both host multi-planet systems. 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.
"Such weak signals of planets
almost the size of the Earth cannot be seen without using advanced statistical
and modeling approaches. We have introduced new methods to remove the noise in
the data in order to reveal the weak planetary signals," said Dr. Feng.
About HIRES
The
High-Resolution Echelle Spectrometer (HIRES) produces spectra of single objects
at very high spectral resolution, yet covering a wide wavelength range. It does
this by separating the light into many "stripes" of spectra stacked
across a mosaic of three large CCD detectors. HIRES is famous for finding
planets orbiting other stars. Astronomers also use HIRES to study distant
galaxies and quasars, finding clues to the Big Bang.
Authors
- Fabo Feng, Mikko Tuomi, Hugh Jones - University of Hertfordshire, UK
- John Barnes - The Open University, UK
- Guillem Anglada-Escude - Queen Mary University ofLondon, UK
- Steve Vogt - University of California at Santa Cruz, USA
- Paul Butler - Carnegie Institute of Washington, USA
About W. M. Keck Observatory
The
W. M. Keck Observatory operates the most scientifically productive
telescopes on Earth. The two, 10-meter optical/infrared telescopes on
the summit of Maunakea on the Island of Hawaii feature a suite of
advanced instruments including imagers, multi-object spectrographs,
high-resolution spectrographs, integral-field spectrometers, and
world-leading laser guide star adaptive optics systems. The Observatory
is a private 501(c) 3 non-profit organization and a scientific
partnership of the California Institute of Technology, the University of
California, and NASA.
Mari-Ela Chock, Communications Officer
W. M. Keck Observatory
(808) 554-0567
mchock@keck.hawaii.edu
Source: W.M. Keck Observatory
