Astronomers have discovered an Earth-sized exoplanet that seems straight out of a nightmare – one hemisphere is perpetually bathed in lava, while the other is solid rock.
Discovery of HD 63433 d
The new planet, dubbed HD 63433 d, orbits HD 63433, a Sun-like star located just 73 light-years from Earth in the constellation Ursa Major. Using data from NASA’s planet-hunting TESS spacecraft, scientists detected slight dips in the star’s brightness every 15 days, indicating a planet about the size of Earth was regularly passing in front.
Follow-up observations by the Keck Observatory in Hawaii confirmed the TESS findings and allowed scientists to determine more details about HD 63433 d. The results were published this week in a series of papers in the Astrophysical Journal Letters.
“This planet is in many ways an analog to early Earth, with a thicker atmosphere and oceans of lava rather than water,” said lead author Dr. Iska Novak, an exoplanet researcher at the SETI Institute. “Studying worlds like this helps us understand the geophysical and atmospheric forces that shape the evolution of terrestrial planets.”
A Planet of Extremes
HD 63433 d orbits so close to its star that one hemisphere is perpetually roasted under immense heat. Surface temperatures on this day-side hemisphere exceed 1,200°C – hot enough to melt rocks. Novak and her team estimate this lava covers 40-60% of the planet.
“The night side, on the other hand, is dramatically cooler,” Novak explained. Temperatures may plunge below -200°C on the planet’s dark side.
This extreme temperature variation arises because HD 63433 d is tidally locked to its star, showing only one face to it, just as the Moon shows only one face (the near side) to Earth. As a result, the exoplanet has permanent day and night sides rather than experiencing sunrises and sunsets like Earth.
“We’re used to seeing artistic renderings of lava worlds, but this one may actually exist,” Novak said.
Table 1. Key Parameters of HD 63433 d
| Parameter | Value |
|---|---|
| Radius | 0.95 Earth radii |
| Mass | 1.5 Earth masses |
| Orbital Period | 15.2 days |
| Equilibrium Temperature | >1,200°C (day), < -200°C (night) |
| Surface | 40-60% molten lava, remainder solid rock |
Glimpse at Early Earth
The discovery of HD 63433 d comes on the heels of NASA’s recent launch of the James Webb Space Telescope. One major goal for Webb and other next-generation observatories is studying rocky exoplanets to search for biosignatures – signs of life.
Worlds like HD 63433 d can serve as analogs for better understanding the early stages of terrestrial planet evolution. Our own Earth likely featured a similar lava ocean billions of years ago before cooling. Oceans of liquid water later condensed from steam released by volcanoes on the young Earth.
“Seeing a planet still trapped in such an early stage of development is extremely exciting,” said Dr. Elisa Quintana of NASA Goddard Space Flight Center, a co-author of the discovery paper. “This planet probably resembles an early Earth, with many active volcanoes and a dense atmosphere.”
Quintana leads a separate TESS science team focused on finding young planets orbiting adolescent stars. She said the priority is finding smaller, rocky worlds rather than more massive gas giants.
“As their stars mature, we’ll be able to study how the planets evolve over hundreds of millions of years,” Quintana noted.
What’s Next
While Webb and other telescopes will collect more data on HD 63433 d’s extreme climate, Novak doesn’t expect follow-up studies to reveal any signs of life. Temperatures are far too hostile for that.
But she’s thrilled that TESS, launched just 6 years ago, has already revealed an exoplanet so alien yet so similar to our early Earth.
“TESS was designed to discover small worlds orbiting bright, nearby stars for exactly this purpose – worlds upon which Webb and future telescopes could shed new light, so to speak,” Novak said. “With more observing time from TESS, I expect we’ll find many more lava planets like HD 63433 d.”
A Glimpse of Earth’s Distant Future?
Some astronomers see HD 63433 d as not only an analog for early Earth, but also a preview of our planet’s own distant fate.
“In 4 or 5 billion years, the Sun will run out of hydrogen fuel and grow into a red giant star,” said Dr. Jim Fuller, an astrophysicist at Caltech unaffiliated with the new study.
The expanding Sun will likely engulf Mercury and Venus and perhaps Earth. But even if our planet escapes that fiery demise, Fuller says the dying Sun will still torch Earth’s surface.
“It will be a one-sided roasting, not unlike what we see on this new lava world,” he said. “So while obviously uninhabitable, planets like HD 63433 d offer useful insights for models of Earth’s very long-term climate evolution.”
Of course, 5 billion years is nearly inconceivable compared to a human lifetime. Still, Novak found poetic significance in studying exoplanets like HD 63433 d.
“Every world has a story to tell,” she said. “This one speaks of beginnings and endings.”
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