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Astrophysics

Worlds like ours

A new generation of spectrographs should permit the discovery of planets that are Earth twins

A new phase in the search for rocky planets resembling Earth outside the solar system, which are known as terrestrial exoplanets, is expected to begin next year as two cutting-edge spectrographs with similar names come online: the American Extreme Precision Spectrograph (EXPRES), and the European Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO). The former is in the final stages of testing in the United States, and the latter is being installed in Chile. This type of instrument separates the light emitted by stars into its different wavelengths and permits the study of some physical and chemical characteristics of celestial objects, and can determine their relative movement in space. With EXPRES and ESPRESSO, astrophysicists expect to acquire an unprecedented ability to find and study Earth twins around nearby stars similar to the Sun.

An ideal system would be composed of a terrestrial exoplanet with mass and size equal or nearly equal to Earth, located approximately the same distance from its star that our planet is in relation to the Sun; in other words, in the habitable zone where there might be liquid water, a prerequisite for the existence of life. Although more than 3,500 worlds outside our solar system have been discovered in the last 20 years and there are another 4,500 potential exoplanets, fewer than a dozen display a good degree of similarity with Earth. For now, according to the catalog created and periodically updated by the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, the exoplanet which most resembles Earth orbits the star Proxima Centauri, which is closest to us at 4.2 light-years away (see table above).

If they function as expected, the new spectrographs will have a resolution 10 times greater than the best instruments operating today, and should measure the faint gravitational effect caused periodically by an Earth twin orbiting around its star—something like a small jolt that has a tiny effect on the trajectory and the speed with which the star moves away from or approaches Earth, known as the radial velocity. This type of disturbance was measured using the Doppler effect in 1995 by astrophysicists at the Geneva Observatory in Switzerland who discovered the first exoplanet orbiting a Sun-type star, a gaseous giant with half the mass of Jupiter located very close to its star.

The problem is that small rocky worlds like Earth cause gravitational wobbles in stars like the Sun which are so tiny that not even today’s best spectrographs can capture them. The most powerful of these devices operating today is the HARPS spectrograph at La Silla Observatory in southern Chile, which is operated by the European Southern Observatory (ESO); it registers variations as small as 1 meter per second (m/s) in the radial velocity of stars. “Earth would not be found by an alien using our current technology,” says astrophysicist Debra Fischer of Yale University, EXPRES project leader. “They would only discover the larger planets in the solar system.”

The mass of Jupiter, the largest planet in the solar system, is 317 times the mass of Earth. Its gravitational pull causes a Doppler effect of 13 m/s on the Sun. The Earth’s gravity causes a much more subtle disruption: it makes the radial velocity of the Sun oscillate 10 centimeters per second (cm/s) for each full revolution Earth makes around its star. Planets similar to Earth which lie outside the solar system should cause similar disturbances of a few cm/s in their parent star’s orbit; this is the level of resolution that the EXPRES and ESPRESSO spectrographs need to reach in order to be useful in the search for terrestrial exoplanets.

The smaller the distance and the greater the mass of a planet in relation to its star, the greater the variation in the star’s radial velocity. “We want to find rocky planets within the zone of habitability of their stars,” says astrophysicist Francesco Pepe of the Geneva Observatory, the coordinator of the ESPRESSO project who also worked on developing HARPS. ESPRESSO is an initiative by Switzerland, Italy, Spain, and Portugal in collaboration with the ESO. It is being installed on the Very Large Telescope (VLT), a set of four main telescopes (each with an 8.2 m mirror) located at the ESO site in Cerro Paranal, Chile. “ESPRESSO can work with only one or even up to four VLT telescopes,” explains Pepe. The estimated cost of the spectrograph is €23 million.

Fischer, who coauthored the discovery of the first system with more than one exoplanet in 1999, was in São Paulo in September of this year to attend the annual meeting of the Brazilian Astronomical Society (SAB). In her presentation, she spoke of the search for terrestrial exoplanets and EXPRES, which cost roughly US$5.2 million. By the end of October, the spectrograph should be fully assembled on the Discovery Channel Telescope, which has a 4.3 m mirror and is located at the Lowell Observatory in Arizona. “First light for EXPRES is scheduled for December 9,” says Fischer, who has worked on a project to find 100 Earth-like exoplanets in the vicinity of the solar system since 2014. Discovery is more modest than the VLT telescope, which can gather measurements from stars that are less bright. But the developers of EXPRES hope to make up for this disadvantage by adopting a more flexible timetable to use their spectrograph than the one used by ESPRESSO in the VLT, which is one of the most contested telescopes in the world. “More time available to conduct observations is a key differential,” says astrophysicist José Dias do Nascimento Júnior of the Federal University of Rio Grande do Norte (UFRN), who was responsible for Fischer’s visit to the SAB meeting. “Sometimes you have to follow an exoplanet for two years to determine its orbit.”

The EXPRES will only cover the sky in the Northern Hemisphere. Fischer plans to have a clone of the spectrograph installed on a telescope in the Southern Hemisphere. “The SOAR [telescope] in Chile is very similar to the Discovery, and it would be easy to install a copy of the EXPRES on it,” she added. The Southern Astrophysical Research telescope has a 4.1 m mirror and was built and is maintained through investments from Brazil, the United States, and Chile. The cost of deploying this replica of the EXPRES (which has been dubbed the Sorceress) is expected to be US$3.6 million. “The instrument is undoubtedly good, and there is initial interest,” says Bruno Vaz Castilho, director of the Brazilian National Astrophysics Laboratory (LNA), which manages Brazil’s participation in SOAR. “But the telescope’s scientific committee has to assess whether it fits in with the operations and future research proposals for SOAR.”

The technological solutions employed in ESPRESSO and EXPRES are different, but they basically attempt to address the same problems in order to achieve the desired accuracy: maintain the optical part of the spectrograph in a vacuum environment with controlled pressure and temperature (approximately -200 Kelvin). The connection between the instruments and the telescopes where they are being installed utilizes a network of optical fibers. “Other latest-generation spectrographs are being built, but only ours and the EXPRES have publicly stated the goal of measuring variations in the radial velocity of stars on the order of 10 cm/s,” says Pepe of the ESPRESSO project. To reach this level of accuracy, the devices must be able to distinguish typical instabilities on the surface of the stars—bubbling and magnetized craters with gases that move hundreds of meters per second—from the tenuous 10 cm/s gravitational shake a planet like Earth makes on the orbit of its star.

Complementary techniques
Astrophysicists dream of obtaining direct images, preferably in the field of visible light, of the new worlds they look for around stars other than the Sun. But stars are so bright that they overshadow any exoplanets surrounding them. New super telescopes that are expected to start operating in 2020, like the Giant Magellan Telescope (GMT) and the European Extremely Large Telescope (E-ELT), may reach this goal. For now, barely more than 1% of the 3,510 extrasolar planets discovered in the vicinity of 2,615 stars have been identified by obtaining low-resolution images of these hidden celestial objects. In almost 99% of cases, the presence of extrasolar planets was determined by the indirect effects these celestial objects have on their stars. The gravitational microlensing technique, which measures changes in the curvature of light, was responsible for the discovery of just over 1% of extrasolar planets.

Between 1995 and 2010, most exoplanets were discovered using the radial velocity technique. In recent years, as missions such as the French CoRoT and especially the American Kepler satellites were dispatched to search for exoplanets, the transit method became most productive in terms of the number of exoplanets found. This method measures the decrease in brightness caused when a planet passes in front of its star, in an event that resembles a mini-eclipse. Today 18% of all confirmed exoplanets were discovered using the radial velocity technique, and 78% using transit. However, both methods share a common bias: they tend to more easily find giant exoplanets which are often gaseous, or at best, rocky worlds larger than Earth.

But the two techniques are not just competing approaches, they complement each other. “Radial velocity can determine the minimum mass an exoplanet can have,” explains Eduardo Janot Pacheco of the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo (IAG-USP), which coordinated Brazil’s participation in CoRoT; Pacheco is the founder of the recently created Brazilian Society of Astrobiology. “The transit provides the size, the diameter, of the exoplanet. One method helps confirm the discovery made by the other and refine the data.” With these two parameters, mass and volume, the approximate density of an object can be calculated to get a sense of whether an exoplanet is gaseous or solid. There is no lack of old and new partners that use the transit method to double up with EXPRES and ESPRESSO, or even other spectrographs. In 2018, NASA is expected to launch the TESS satellite, Kepler’s successor. Over the next decade, the European Space Agency (ESA) expects to send the Plato mission to space, which will use the transit method to search for terrestrial exoplanets.

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