How do astronomers find exoplanets?  5 of the best methods, explained

How do astronomers find exoplanets? 5 of the best methods, explained

March 21, NASA announced the confirmation of the 5000th planet outside our Solar System. From scorching gas giants nestled close to their parent star to rocky worlds that may harbor water on their surface, there is a variety for scientists to study.

But finding these strange new worlds is a science in itself. We’ve only been able to definitively detect planets of any kind for a few decades, and even then, there are challenges in detecting such a small object at that distance even in the most powerful telescopes.

Reverse spoke with Marie-Eve Naud, exoplanet researcher and outreach coordinator at the Exoplanet Research Institute at the University of Montreal, to tell us more about how astronomers find these worlds and the considerations for each method. While there are numerous methods, the ones listed below are the most common.

Planets can cause mini-eclipses that reveal their presence to observers on Earth. POT

The transit method

Astronomers have found most known exoplanets through the transit method, first demonstrated in 1999 to confirm the existence of planet HD 209458b. NASA’s Kepler Space Telescope, launched in 2009, detected thousands of planets in a fixed area of ​​the sky near the constellation Cygnus. As the planets passed across the face of their star, they dimmed the star’s light slightly. These changes in light levels are detectable by instruments called photometers.

For this method to work, astronomers must detect a small change in the star’s inherent brightness (luminosity), often less than one percent. In space, this is easier to achieve due to the lack of atmosphere that interferes with observations and is the preferred method for missions such as the European Space Agency’s Exoplanet Characterization Satellite (Cheops) and the Exoplanet Survey Satellite. in Transit (TESS) from NASA.

Naud cautioned that multiple detections are required, meaning a planet must be viewed through multiple orbits to ensure dimmed light isn’t due to sunspots or dust. “Usually you wait until you’ve seen two or three transits,” he said, emphasizing that astronomers want a lot of data from the transit method before confirming that a planet is there.

Once a planet is detected, astronomers can estimate its radius. Astronomers must find the mass separately (often through the radial velocity method, detailed below). If astronomers can determine a planet’s mass and radius, they can determine whether it is likely to be rocky or gaseous, which has significant implications for life.

If they find that the planet is rocky and in an area of ​​the parent star that can hold water, for example, they could consider it habitable. But there are complications, such as if the planet is close to a star that often erupts, like a red dwarf. The constant eruptions bathe a world in radiation and threaten any emerging microbes. That’s why astronomers aren’t sure if TRAPPIST-1, for example, hosts habitable exoplanets even though astronomers have found seven Earth-sized exoplanets in this system, and some are in the habitable zone.

Some planets are invisible to us and we need to confirm their existence by other means, including their gravitational pull. POT

radial velocity method

Radial velocity is a common way of finding planets, especially with observatories such as the High Precision Radial Velocity Planet Finder (HARPS) instrument on the European Southern Observatory’s 3.6m La Silla telescope in Chile.

“We often say that the planet orbits the star, but in fact, both objects are based around their center of mass,” Naud explained. “That star moves slightly, if it has a planet. If you have many, you will have a complex movement.”

The key is to look at the spectrum of the star. As the star moves towards us, its light is compressed and the spectrum changes to red. As the star recedes, its light is stretched and the spectrum changes to blue. The star’s spectrum is slightly affected by the planet’s motion, making it “a kind of barcode of the star,” Naud explained.

The first detection of a planet around a Sun-like star with this method occurred in 1995 when Didier Queloz and Michel Mayor published their findings in 51 Pegasi b. But it wasn’t the first detection of a planet: Two were reported around the pulsar PSR B1257+12 in 1992, and a third planet was confirmed in 1994. Astronomers Aleksander Wolszczan and Dale Frail detected the planets through intermittent changes in radio of the pulsar. signal, corresponding to an orbiting planet passing between the pulsar and Earth. It shows you the variety of scientific means by which we can find different worlds.

Four planets in the HR 8799 system have been directly imaged, all more massive than Jupiter. Jason Wang (Caltech)/Christian Marois (NRC Herzberg)

direct images

Direct imaging allows scientists to remove the light from a star to see exoplanets directly. You may think this is an easy method, but it’s more complicated than it seems. Since these planets are quite small and have relatively little reflected light for a telescope to see, the technique remains difficult for astronomers to achieve with current telescope technology. That’s why astronomers have only found about 60 planets with this technique so far.

Still, there are benefits: “It’s the only method that allows direct detection of exoplanets,” Naud said. While these planets are points of light for now, we may be able to see more detail in their atmospheres and on their surfaces in the distant future.

There are two main methods scientists use for direct imaging today. The first, coronagraphy, blocks light from the star’s outermost layer (or corona) using a device inside the telescope called a coronagraph to create an artificial eclipse to detect planets.

For example, Canada’s Gemini Planet Imager is installed on the 8.1-meter telescope at the Gemini South Observatory, Cerro Pachón, Chile. Using a coronagraph, he found the first known “young Jupiter,” 51 Eri b, in August 2015. (Naud said GPI is considered a pioneer in direct imaging.)

The second method is a star screen, which blocks a star’s light before it moves into the telescope. Star screens designed for exoplanets must be a spacecraft separate from a telescope to ensure they are at the right distance and angle to effectively block starlight, unlike star screens that are used to shield against light. of our star, like the one built into the James Webb Space Telescope. To date, a stellar shadow has not flown into space.

Hubble has detected some microlensing exoplanets, and the James Webb Space Telescope may have a chance to do the same.POT

gravitational microlensing

Although astronomers have detected a significant minority of planets (more than 130 at this time) with a technique called microlensing, the observations are difficult to follow. The problem is that we often see these worlds only once. This is because they use a phenomenon, first described by Einstein, in which a large foreground object (such as a star or galaxy) deflects light from a much smaller object behind. Since these alignments in the sky are brief and unpredictable, this often means we can’t re-examine a particular planet.

“It’s a bit frustrating, because it happens once when you have perfect alignment with a background object,” Naud said, cautioning that it’s challenging to set the work up for future studies “because we’ve only seen it that one time.”

The alignment changes slightly when a planet, which has its own gravity, is involved. The planet’s gravity adds an additional lensing event to the star’s light, brightening the image of the background star and allowing astronomers to learn much about that planet, including its mass and orbital period despite the event’s fleeting nature. .

Astronomers first detected a planet using this method in 2003. The detection combined the capabilities of the Polish Optical Gravitational Lensing Experiment (OGLE), which examines stellar variability, with the Microlensing Observations in Astrophysics (MOA) of Japan and New Zeeland. The microlensing event is known as 2003-BLG-235 and was probably a planet 1.5 times the mass of Jupiter.

The most distant confirmed microlensing planet is MOA-2011-BLG-291L b, located 28,700 light-years away. However, there is circumstantial evidence of planets outside our galaxy, but because microlensing events are so fleeting, it is impossible to confirm them.

Astrometry is based on changes in a star’s position caused by a planet. European Space Agency

astrometry

Astrometry tracks offsets in a star’s position produced by an orbiting exoplanet. Because stars and their planets orbit around a barycenter, astronomers can detect a star’s wobble caused by a planet with extremely precise instruments. The technique is quite difficult as it relies on even more precise observations than radial velocity and has been the source of several controversial claims.

A notorious example of the difficulties of astrometry in discovering exoplanets occurred in 1963. Swarthmore College astronomer Peter van de Kamp announced that he had found a planet around Barnard’s star using Swarthmore’s 24-inch refractor. at the Sproul Observatory. It turned out that adjustments to the telescope’s main mirror had produced a false signal and that exoplanets did not exist.

The European Space Agency’s Gaia mission, which accurately plots the positions of stars from space, is expected to use astrometry to search for exoplanets. The next data release, scheduled for later this year, may include the first astrometric detections of exoplanets.

NASA’s exoplanet archive only lists one planet found by astrometry: DENIS-P J082303.1-491201b, also known as VB 10b. However, since the planet (or brown dwarf, depending on the source consulted) did not appear in the follow-up radial velocity observations, “most researchers consider it to be another false positive,” according to the Planetary Society. That’s because scientists generally prefer to have at least two detections of a planet to confirm its existence.

The future of exoplanet hunting

While planet detections in the near future are likely to use variations of the above methods, some more advanced ideas may allow us to look at other worlds in more detail. One idea, for example, is to use the Sun as a gravitational lens. However, a spacecraft would need to travel 550 Sun-Earth distances to use it effectively, which is seven to eight times farther from Earth than the Voyager spacecraft.

On the ground, scientists eagerly await first light from the Extremely Large Telescope, a European observatory that is expected to take direct images of some planets and better characterize their atmospheres. It is scheduled to start operating in 2027.

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