- Astronomers have announced evidence for a possible planet in another galaxy.
- This “exoplanetAn exoplanet (or extrasolar planet) is a planet that is outside the Solar System, orbiting around a star other than the Sun. The first suspected scientific detection of an exoplanet occurred in 1988, with the first confirmation of detection coming in 1992.”>exoplanet” would be much farther away than any of the thousands of others scientists have found in our Milky WayThe Milky Way is the galaxy that contains the Earth, and is named for its appearance from Earth. It is a barred spiral galaxy that contains an estimated 100-400 billion stars and has a diameter between 150,000 and 200,000 light-years.”>Milky Way Galaxy in recent years.
- This planet candidate was identified with NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It’s vision is “To discover and expand knowledge for the benefit of humanity.””>NASA’s Chandra X-ray Observatory that detected a temporary dimming in X-rays in a binary system.
- Researchers interpret this dimming as a planet passing in front of an X-ray source around a neutron starA neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.”>neutron star or black holeA black hole is a place in space where the pull of gravity is so strong not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.”>black hole orbiting a companion star.
Astronomers have found evidence for a possible planet candidate in the M51 (“Whirlpool”) galaxy, potentially representing what would be the first planet seen to transit a star outside of the Milky Way. Researchers used NASA’s Chandra X-ray Observatory to detect the dimming of X-rays from an “X-ray binary”, a system where a Sun-like star is in orbit around a neutron star or black hole. The authors interpret this dimming as being a planet passing in front of the neutron star or black hole.
The left panel of this graphic shows M51 in X-rays from Chandra (purple and blue) and optical light from NASA’s Hubble Space TelescopeThe Hubble Space Telescope (often referred to as Hubble or HST) is one of NASA’s Great Observatories and was launched into low Earth orbit in 1990. It is one of the largest and most versatile space telescopes in use and features a 2.4-meter mirror and four main instruments that observe in the ultraviolet, visible, and near-infrared regions of the electromagnetic spectrum. It was named after astronomer Edwin Hubble.”>Hubble Space Telescope (red, green, and blue). A box marks the location of the possible planet candidate, an X-ray binary known as M51-ULS-1. An artist’s illustration in the right panel depicts the X-ray binary and possible planet. Material from the companion star (white and blue in illustration) is pulled onto the neutron star or black hole, forming a disk around the dense object (illustrated as red and orange). The material near the dense object becomes superheated, causing it to glow in X-ray light (white). The planet is shown beginning to pass in front of this source of X-rays.
Looking for the dimming of a star’s light as something passes in front of it is called the transit technique. For years, scientists have discovered exoplanets using transits with optical light telescopes, which detect the range of light humans can see with their eyes and more. This includes both ground-based telescopes and space-based ones like NASA’s Kepler mission. These optical light transit detections require very high levels of sensitivity because the planet is much smaller than the star it passes in front of, and, therefore, only a tiny fraction of the light is blocked.
The scenario of a transit in an X-ray binary is different. Because a potential planet is close in size to the X-ray source around the neutron star or black hole, a transiting planet passing along Earth’s line of sight could temporarily block most or all of the X-rays. This makes it possible to spot transits at greater distances — including beyond the Milky Way — than current optical light studies using transits. A graphic (above) shows how X-rays from M51-ULS-1 temporarily decrease to zero during the Chandra observations.
While this is a tantalizing study, the case of an exoplanet in M51 is not ironclad. One challenge is that the planet candidate’s large orbit in M51-ULS-1 means it would not cross in front of its binary partner again for about 70 years, thwarting any attempts for a confirming observation for decades. There is also the possibility that the dimming of X-rays is due to a passing cloud of gas near the M51-ULS-1, though the researchers think the data strongly favor the planet explanation.
Reference: “A possible planet candidate in an external galaxy detected through X-ray transit” 25 October 2021, Nature Astronomy.
The paper describing these results appears in the latest issue of Nature Astronomy. The authors are Rosanne DiStefano (CfAThe Harvard-Smithsonian Center for Astrophysics (CfA) is a joint venture between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. Founded in 1973, the Harvard-Smithsonian Center for Astrophysics is comprised of six research divisions: Atomic and Molecular Physics; Optical and Infrared Astronomy; High Energy Astrophysics; Radio and Geoastronomy; Stellar, Solar, and Planetary Sciences; and Theoretical Astrophysics.”>CfA), Julia Berndtsson (Princeton), Ryan Urquhart (Michigan State University), Roberto Soria (University of the Chinese Science Academy), Vinay Kashap (CfA), Theron Carmichael (CfA), and Nia Imara (now at the University of California at Santa Cruz). NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts.