The Oldest Known Planet

In this glorious new era of planet hunting, astronomers have been discovering all sorts of freakish & unexpected worlds. To date scientists have confirmed over 4300 exoplanets (and they have partial data or leads on thousands of others). Among the 4300 known planets there are some true odd balls–a carbon planet nine times the size of Earth made of diamond, a super ghost planet twice the volume of Jupiter with half the mass, “hot Jupiter” worlds which are bigger than Jupiter but closer to their suns than Mercury. Today scientists have announced a whole new superlative: they discovered the oldest known planet, a world which dates back to the roaring infancy of the galaxy. This world goes by the name TOI-561b and, before our sun spun into existence, it was already older than the sun is now.

To put this in proper context, we need some astronomical ages (which are highly revealing in their own right). Here is a useful table of the known ages of various galaxies, stars, and, uh, everything. These dates are in Earth years (in case you are reading this elsewhere, I guess) as of AD 2021. The dates may be subject to some revision, although we keep zeroing in with greater and greater precision so I do not anticipate any large shifts:

Age of Universe	13.77 Billion Years
Age of Milky Way Galaxy	13.4 Billion Years
Age of TOI-561b	10 Billion Years
Age of Sun	4.6 Billion Years
Age of Planet Earth	4.543 billion years
Age of Cher	74 Years

The exoplanet TOI-561b is located in the thick disk of the Milky Way. The planet is close to its home star which it orbits twice an Earth Day (ergo, a TOI-561b year lasts about twelve hours). Because TOI-561b is so close to its star, it is hot, the surface temperature is reckoned to be about 2,000 Kelvin (3,140 degrees Fahrenheit). The rocky world has a volume 50 percent larger than Earth and a mass three times as great. The density of TOI-561b, however, is much less than that of Earth and this is because the planet is largely lacking in metals. The reason for this absence is disconcerting: TOI-561b formed before many stars had exploded, back in a time when metal was rare in the universe. There may not be many planets older than TOI-561b anywhere because there was nothing to make planets out of in the time before it formed. A generation or two of stars needed to live and die before such elements existed!

Thinking of how different things were when Cher was young is hard for me! (I like Cher and I put her up there because she is still a dynamic star). Thinking of what things were like when TOI-561b formed out of the first matter more complex than hydrogen and lithium and whatnot is practically inconceivable! It must have been quite a view though: hovering above a glowing brand-new galaxy sparkling with supernovae. I wonder if there was ever any sort of (non metallic) life there before its star became old and giant and hot.

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