Super-Earths and Mini-Neptunes


  1. Super-Earths and Mini-Neptunes
  2. When your rover needs a rover - 2031 Mars sample return

Today I learned

In investment banking, we analysts dislike fiscal quarter ends because we have to manually update the share counts and cash/debt balances of all the companies we cover. But as a Planetary Society member I have something to look forward to - quarterly editions of the Planetary Report mailed to my apartment! The March edition had a great article on Super-Earths and Mini-Neptunes that I wanted to share!

An artist's impression of a variety of Super-Earths

Over the past 30 years astronomers have discovered thousands of exoplanets, but they're puzzled because a very common type of exoplanet has no counterpart in our own solar system - one that's larger than Earth, the Sun's largest rocky planet, but smaller than Neptune, the smallest of the gas giants at ~4x Earth's diameter. Is this absence a coincidence? We don't know, but clearly at some point in between, planets must make the jump from solid and terrestrial to mostly hydrogen/helium gas giants

There's a vast Universe of exoplanets we've only just begun to understand! | Credit: The Planetary Society

To understand this transition as mass increases, astronomers have measured the absorption spectra of hundreds of exoplanets to try determine their compositions. When an exoplanet passes in front of its star, its atmosphere filters the starlight in a unique way based on the compounds in its atmosphere. They're particularly interested in water vapor, not just because it's common, but also because in our solar system, increased presence of heavy compounds (such as water, or anything bigger than helium) correlates with lower mass - Neptune has about 25x more heavy molecules than Jupiter. But this trend hasn't held so well with exoplanets; there's so many outliers that we can't definitely explain the gradient from Super-Earth to Mini-Neptune.

Apart from water vapor, I bet astronomers are also curious about carbon dioxide and methane | Credit: The Planetary Society

So is it back to the drawing board? Hard to say... but with the recent launch of NASA's Transiting Exoplanet Survey Satellite (TESS) and the highly anticipated launch of the James Webb Space Telescope (Hubble's successor) in 2021, we'll soon have the tools needed to tackle this intriguing question! 

Current events

NASA and the European Space Agency may finally be working on a Mars sample return mission! I'm trying not to get my hopes up, since this is still very preliminary and there's no formal announcement yet, but if it all goes through we'll have our first samples of Martian soil back on Earth by 2031!

I hope this happens, I've been waiting for this for years! NASA needs the funding to get it done!

Here's how it'd work - NASA's next Mars rover (still unnamed) is scheduled to launch in 2020. It'll explore the surface, scoop soil into tightly sealed tubes, and store them somewhere safe. A smaller "fetch" rover will arrive later to bring the samples to the Mars ascent vehicle, which will blast off and rendezvous with an orbiting spacecraft. That spacecraft will then carry the samples back to Earth.

Artist's impression of the Mars Sample Return orbiter in transit back to Earth

Even our most advanced rovers like Curiosity are still very limited in the science they can do compared to the high tech laboratories we have on Earth, so retrieving samples could help us unlock the mysteries of the Red Planet. Obviously this is a highly complex mission with lots of choreography - we'll need some serious robotics and rocket science to make it happen. But thankfully, human ingenuity knows no bounds!

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