A planet as old as Methuselah
By Amanda Jermyn

How old is the oldest known planet? Almost as old as the universe, it turns out. At 12.7 billion years old, planet Psr B1620-26 B is almost three times the age of Earth, which formed some 4.5 billion years ago. This exoplanet, the oldest ever detected in our Milky Way galaxy, has been nicknamed “Methuselah” or “the Genesis planet” on account of its extreme old age.

Methuselah formed about a billion years after the birth of the universe in the Big Bang. According to NASA, the planet has had a remarkable history because it inhabits an unlikely neighborhood, orbiting an unusual pair of burned out stars near the crowded core of a globular cluster of over 100,000 stars. Planetary systems in such environments tend to be ripped apart by gravitational interactions with other nearby stars, yet this planet survived. Methuselah was just the first of a number of planets discovered orbiting two stars, and also the first planet ever found in a globular cluster.

One of the stars Methuselah orbits is a pulsar, a rapidly rotating super-dense neutron star that emits regular pulses of electromagnetic radiation. It was first discovered in 1988 in the M4 globular cluster. The other star is a white dwarf, a small, dense star about the size of a planet that has exhausted its nuclear fuel. The white dwarf was detected through its effect on disrupting the regular pulses of the neutron star as the two stars orbited each other twice a year. In 1993 astronomers detected further irregularities in the pulsar that implied that a third object was orbiting the others. They surmised that it was either a large planet or a brown dwarf, a celestial object with a mass greater than a giant planet but less than that of a small star.

The issue was finally resolved in 2003 when measurements taken by the Hubble space telescope showed that the object is 2.5 times the mass of Jupiter. This confirmed that it is indeed a planet, rather than a brown dwarf. The ancient planet’s existence provides evidence that planets were able to form rapidly, within a billion years of the Big Bang, suggesting that our galaxy may be teeming with such worlds.

At the other end of the spectrum, planet V830 Tau b may be the youngest fully formed exoplanet ever found. V830 Tau b orbits a star that is estimated to be only two million years old, which suggests that the planet itself is of similar age, a mere infant in cosmic terms. With 77% the mass of Jupiter, it orbits its star about every 5 days and is about seven times closer to its star than Mercury is to the Sun. It was discovered by observing the wobble of the star caused by the gravitational pull of the orbiting planet.

Another young planet, K2-33b, was discovered by the Kepler space mission. It was detected using the transit method, by observing how the star dims as the planet passes in front of it. This Neptune-sized planet and its star are estimated to be about five to ten million years old. The planet whips around its star every five days, and is nearly ten times closer to its star than Mercury is to the Sun. Additional data from the Keck Observatory in Hawaii and NASA’s Spitzer Space Telescope show that the star is also orbited by a thin layer of debris. This is likely a remnant of the thick disk of debris that surrounded the star when it first formed, providing the raw material from which new planets form.

As most of the 4,000 confirmed planets beyond our solar system are more than a billion years old, the discovery of these very young planets and their stars provides an opportunity to observe the early stages of planetary development.

Join the Springfield Stars Club on Tuesday, February 25th at 7:00pm at the Springfield Science Museum for a talk by Michael Kerr, the new Principal for Innovative Advancement and Integration in Science at the Springfield Museums. Kerr will talk on “Lasers and Rockets and Stars, Oh My!” Prior to joining the Springfield Museums in 2019 as Curator of Astronomy Kerr worked for many years at the MIT Lincoln Laboratory. His areas of expertize include high-altitude atmospheric and near-earth space phenomenology, laser beam propagation, sensors and signal processing, and missile defense. He earned a B.S. degree in Physics and Astronomy at the University of Rochester and did graduate studies in Physics and Astronomy at the University of Toledo. Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.

Also, on Friday, March 6th at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Richard Sanderson, retired Curator of Physical Science at the Science Museum will give a talk on “Observing the Sky during Sunset and Twilight.”