Exoplanets

An exoplanet, or extrasolar planet, is any planet outside of our solar system (Brennan, 2020; Lissauer, 2020). Most of the known exoplanets are in a relatively small part of our galaxy, the Milky Way (Brennan, 2020). We can know an exoplanet's physical compositions and characteristics by calculating its size and mass (Brennan, 2020). Some planets may rocky like Earth and Venus, while others may be gas-filled like Saturn and Jupiter (Brennan, 2020). The elemental composition of exoplanets are similar to those of planets in our solar system, but their combination of elements may be different; some planets may be predominantly carbon or iron, while others are primarily water or ice (Brennan, 2020).

First Confirmed Discovery: PSR 1257+12 Planets

On January 9, 1992, astronomers Aleksander Wolszczan and Dale Frail discovered the first exoplanets (Wenz, 2019). They found two planets orbiting the pulsar PSR 1257+12, 2700 light-years away (Wenz, 2019). Pulsars emit pulses of electromagnetic waves, and some pulse at such a steady rate that "they are considered the most accurate natural clocks in the universe" (Cofield, 2016). PSR 1257+12 should have pulsed every 6.219 milliseconds, but occasionally, some pulses were slightly off; however, those anomalies came at consistent intervals (Wenz, 2019). By studying variations in the pulses of PSR 1257+12 using the 305 meters, now collapsed Arecibo radio telescope, Wolszczan and Frail (1992) determined that the pulsar is orbited by two planets. They found that the two planets have masses of 2.8 and 3.4 times the mass of Earth, have distances of 0.47 AU and 0.36 AU from the pulsar, and completes an orbit in 98.2 and 66.6 days, respectively.

Wolszczan (1994) soon recognized that "the approximately 3:2 ratio of the planets' orbital periods should lead to accurately predictable and possibly measurable gravitational perturbations of their orbits." After three years of methodically timed observations of PSR 1257+12, Wolszczan concluded that the clear realization of this gravitational effect, along with the identification of a moon-mass object orbiting the pulsar, provides undeniable evidence that the first exoplanets were discovered.

Detection Methods

It is very uncommon for an astronomer to see an exoplanet through a telescope; most exoplanets are detected using indirect methods (Brennan, 2020).

Transit Photometry Method

The transit photometry method is responsible for most exoplanet discoveries. Currently, it is the most It relies on the fact that stars seem faintly dimmer when a dimmer object passes in front of them. The transit method involves regularly measuring a star's brightness to identify the recurring dimming correlated to an exoplanet's transits (Institution for Research on Exoplanets [iREx], n.d.). In astronomy, a transit is the passing of a celestial object in front of another that is relatively large, but unlike in an eclipse, the smaller object only covers a tiny portion of the larger one (Dunbar, 2017; The Editors of Encyclopedia, 2016). If such a decrease in luminosity occurs periodically and lasts a set duration, then it is highly probable that a dimmer body is orbiting the star (The Planetary Society, 2020).

Here, a transit refers to a planet passing in front of a star. These transiting bodies could be smaller, dimmer stars, in which case the two stars are known as eclipsing binaries, but they are mostly planets (The Planetary Society, 2020). The transit of a Jupiter-sized planet would cause a perceived brightness decrease of about 1% for a Sun-sized star, while an Earth-sized planet would cause a brightness decrease of about 0.001% (iREx, n.d.).

Wobble Method

The wobble method consists of

Detection Devices

Transiting Exoplanet Survey Satellite

TESS’s main purpose is to observe distant stars looking for irregular patterns in shadows on the star's surface. When two stars are orbiting each other it is called a binary star system. In a binary star system, one star is a small cool M dwarf and one is a star similar to our sun. M-type stars are also known as red dwarfs, their luminosity is dim and tends to glow red. Also, they have a fairly long lifetime as they burn their hydrogen slowly. Together, these stars form what is known as an eclipsing binary. TESS’s main purpose is to look to these systems and other types of systems and look for tiny regular dips in starlight called transits.

Examples of Exoplanets

PSR 1257+12 Planets

The planets of the PSR 1257+12 pulsar were the first exoplanets discovered (Wenz, 2019). The discovery is covered in the #First Confirmed Discovery: PSR 1257+12 Planets section above.

TOI 1338 b

In 2019, Wolf Cukier, a 17-year-old high school student who was attending NASA’s Goddard Space Flight Center in Maryland for a summer internship, was tasked with examining variations in star brightnesses captured by NASA’s Transiting Exoplanet Survey Satellite (TESS) (Kazmierczak, 2020). Cukier was looking through records of eclipsing binaries when, on the third day, he noticed a signal from TOI 1338. He initially thought indicated a stellar eclipse, but the timing was incorrect (Kazmierczak, 2020). Ultimately, Cukier was able to distinguish the planetary transits from the more frequent instances of eclipsing binaries, which can have similar-looking data (Nasa Goddard, 2020). Named TOI 1338 b, the planet was the first circumbinary planet—a planet that orbits two stars—detected by TESS (Kazmierczak, 2020). It is between the size of Neptune and Saturn and has an irregular transit, orbiting its two stars every 93 to 95 days (Kazmierczak, 2020).

References

Brennan, P. (2020, December 03). What is an Exoplanet? Exoplanet Exploration: Planets Beyond Our Solar System. https://exoplanets.nasa.gov/what-is-an-exoplanet/overview/

Cofield, C. (2016, April 22). What are pulsars? Space.com. https://www.space.com/32661-pulsars.html

Dunbar, B. (2017, August 3). Eclipses and transits: Overview. Nasa. https://www.nasa.gov/content/eclipses-and-transits-overview

Institute for Research on Exoplanets. (n.d.). Transit method. http://www.exoplanetes.umontreal.ca/transit-method/?lang=en

Kazmierczak, J. (2020, January 6). Nasa's first TESS mission uncovers its 1st world with two starts. Nasa.

https://www.nasa.gov/feature/goddard/2020/nasa-s-tess-mission-uncovers-its-1st-world-with-two-stars

Lissauer, J. J. (2020, February 12). Extrasolar planet. In Encyclopedia Britannica. https://www.britannica.com/science/extrasolar-planet

NASA Goddard. (2020, January 6). TESS satellite discovered its 1st world orbiting 2 stars [Video]. YouTube. https://www.youtube.com/watch?v=8FrlhrtVEW8

The Editors of Encyclopedia Britannica. (2016, May 12). Transit. In Encyclopedia Britannica. https://www.britannica.com/science/transit-astronomy

The Planetary Society. (2020, February). Down in front!: The transit photometry method. https://www.planetary.org/articles/down-in-front-the-transit-photometry-method

Wenz, J. (2019, October 8). How the first exoplanets were discovered. Astronomy. https://astronomy.com/news/2019/10/how-the-first-exoplanets-were-discovered

Wolszczan, A. (1994). Confirmation of Earth-mass planets orbiting the millisecond pulsar PSR1257 + 12. American Association for the Advancement of Science, 264(5158), 538-542.

https://doi.org/10.1126/science.264.5158.538

Wolszczan, A., & Frail, D. A. (2006). A planetary system around the millisecond pulsar PSR1257 + 12. Nature, 355, 145-147. https://doi.org/10.1038/355145a0