Exploring the Realm Beyond: The Quest for Exoplanets
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Exoplanets come in a variety of sizes and orbital patterns. Some feature vast gaseous atmospheres, while others possess rocky or icy surfaces. "In general, exoplanets can be any size, and they are found in a wide range of orbits," says Debra Fischer.
To date, over 4,000 exoplanets have been identified. Roughly one-third of these are gas giants, another third are akin to Neptune, and the remaining third includes Super Earths, with a few being terrestrial.
Gas giants and Neptune-like planets mainly comprise gaseous atmospheres with no discernible surface. Super Earths differ significantly from the rocky planets in our solar system; they are heavier than our terrestrial planets but lighter than gas giants, and their surfaces could be rocky or predominantly gaseous.
The relatively few terrestrial planets found may be misleading; they may not be rare, but rather difficult to detect due to their small size, as will be discussed later.
Our Solar System's Uniqueness
An analysis of the distribution of planetary masses against their distance from their host stars reveals that many exoplanets do not share similarities with those in our solar system. This suggests that the arrangement of planets in our solar system is somewhat unusual.
The accompanying scatter plot illustrates that most discovered planets are located closer to their stars than Earth is to the Sun (1 AU). Some are even situated closer than Mercury, which orbits at 0.39 AU. Many planetary systems feature planets that orbit significantly nearer to their stars.
Additionally, most of the identified planets have masses starting from around 0.01 Jupiter masses. In contrast, our solar system hosts very few planets with masses comparable to our rocky planets (0.002 Jupiter masses or below).
The primary reason for this disparity is that detecting more massive planets has proven to be easier. This aspect will be elaborated upon in the subsequent sections regarding discovery methods.
Hot Jupiters: Gas Giants in Tight Orbits
Why are so many Jupiter-like gas giants found in close orbits around their stars, while the gas giants in our solar system are located farther out? Known as Hot Jupiters, these planets have orbits that are even smaller than Mercury's.
It's noteworthy that most planets with orbits shorter than 1 AU have been discovered using the transit method. This method is particularly effective for detecting planets near their host stars.
However, one must wonder why gas giants are absent close to the Sun if that seems to be a common occurrence in other systems. It could be that our Jupiter did not end up closer to the Sun during the formation of the planetary disk, allowing rocky planets like Earth to exist.
Discovery Techniques
How have scientists discovered the exoplanets we know today? Various methods exist, and a planet must be confirmed through at least two techniques to be included in the official catalog.
#### Radial Velocity Method
In a two-body system, a planet orbits a star, and both bodies revolve around their common center of mass. This results in the star exhibiting a slight movement, causing a wobble that creates a Doppler effect in the light waves reaching us.
When the star moves away from us, the wavelengths are redshifted; when it moves toward us, they are blueshifted. This radial velocity method is how most exoplanets were initially discovered, as seen by the red marks in the first plot.
The extent of this movement is dependent on the mass of the planet relative to the star. For example, Earth's effect on the Sun results in a movement of just 0.1 m/s annually, while Jupiter's influence causes a shift of 12.4 m/s over its 12-year orbital period.
To detect smaller planets, extremely sensitive instruments are required, which is why early discoveries were predominantly massive gas giants. The first confirmed exoplanet was 51 Pegasi b, identified in 1995, although there were ambiguous discoveries prior to that.
51 Pegasi b is classified as a Hot Jupiter, orbiting its star in just about 4 days.
#### Transit Method
The transit method involves measuring the light intensity from a star. If a planet transits in front of the star, it blocks a fraction of the starlight, resulting in a dip in the light flux.
The maximum flux occurs just before the planet obscures the star, allowing scientists to infer the presence of an orbiting planet if they observe periodic fluctuations in light intensity.
Only planets whose orbits cross our line of sight can be detected this way. Planets closer to the star will block more light, which explains the abundance of smaller separation planets found via this method.
In 2014, there was a surge in discoveries due to the Kepler mission, which confirmed 715 new exoplanets, including many Earth-sized ones.
By combining the transit method with spectroscopy, researchers can analyze the composition of a planet's atmosphere. Different molecules absorb and emit light at distinct wavelengths, and this information is gleaned from the light that passes through the planet's atmosphere.
#### Imaging Techniques
While the radial velocity and transit methods have been the primary means of discovery, advancements in camera technology have enabled direct imaging of exoplanets. This technique has only resulted in a handful of discoveries, with the first occurring in 2004.
Currently, this method is limited to large exoplanets that are located at significant distances from their stars.
#### Microlensing
The microlensing technique utilizes the gravitational bending of light, as described by Einstein's theory of relativity. When a star passes in front of a distant object, its gravity bends and magnifies the light from the background object.
If the star hosts a planet, there will be a slight variation in brightness in the magnified object, detectable only during a specific alignment of stars.
This method relies on precise positioning and timing, which can limit the number of planetary discoveries made through it.
Looking Ahead
Innovative telescopes and space missions are in development as the quest for Earth-like planets and potential extraterrestrial life accelerates. The Hubble, Spitzer, and Kepler/K2 telescopes have played significant roles in current exoplanet discoveries.
"Hubble is the most important telescope in history after Galileo’s first telescope," remarks Sandra Faber.
NASA's TESS, launched in 2018, employs the transit method over an area 400 times larger than Kepler's. The mission aims to find 30,000 additional exoplanets over its two-year duration.
#### Upcoming Missions
CHEOPS (Characterising ExOPlanet Satellite) is ESA's upcoming mission set to launch later this year, focusing on the detailed characterization of smaller Earth-Neptune sized planets and their atmospheres.
While current methods provide some insights into atmospheric compositions, CHEOPS will enhance our understanding of essential molecules like carbon dioxide and methane, which emit light in the infrared spectrum. The ultimate goal is to identify planets with Earth-like atmospheres.
The James Webb Space Telescope (JWST) is anticipated to launch soon, potentially in 2021, although its launch has faced multiple delays. The JWST will enable detailed measurements of planetary atmospheres and investigate the universe's history.
"It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang to the evolution of our own Solar System," NASA states regarding the JWST.
PLATO (PLAnetary Transits and Oscillations of stars) is scheduled for a 2026 launch by ESA, with a focus on discovering Earth-like planets in habitable zones around Sun-like stars.
ARIEL, another ESA mission set for 2028, aims to observe around 1,000 known exoplanets to further study their chemical compositions and thermal properties.
We are indeed living in an exciting era of astronomical discovery!