Sciency Words: Earth Similarity Index

March 3, 2017

Sciency Words BIO copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


As I’m sure you’ve heard by now, astronomers have discovered seven planets orbiting a nearby star called TRAPPIST-1. Even more exciting, most or all of these new worlds are being described as Earth-like planets. But what does that actually mean?

You’d be surprised by how many “Earth-like” planets/moons we have right here in our own Solar System.

Ag16 Earth-like Worlds

From left to right: Venus, Earth, Mars, and Titan (Saturn’s largest moon).

Earth-like is a rather vaguely defined term. So in 2011, a paper published in the journal Astrobiology attempted to establish an official mathematical system for calculating just how Earth-like an exoplanet is. It’s called the Earth Similarity Index or E.S.I.

Basically, the E.S.I. takes certain characteristics of a planet that can be quantified—such as a planet’s mass, radius, temperature, etc—and compares them to Earth’s. An E.S.I. score of zero indicates a planet that has absolutely nothing in common with Earth, while an E.S.I. of one means the planet is an exact match for Earth… at least with regard to the characteristics being measured and included in our calculations.

Of course even a planet with an E.S.I. of one is not necessarily habitable, so the same Astrobiology paper also proposes a Potential Habitability Index or P.H.I. But that, I think, is a Sciency Word for another day.

P.S.: If you want to dive into the math behind the E.S.I., click here.

Sciency Words: Apollos and Atens

November 25, 2016

Sciency Words BIO copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today we’ve got two terms:


Asteroid are classified into different “groups” based on their orbital properties. The Apollo asteroids and Aten asteroids are two such groups, and these groups are of particular interest to anyone who doesn’t want a repeat of the K-T Event (which wiped out the dinosaurs) or the Tunguska Event (which flattened a forest and could have done the same to a whole city).

Technical Definitions

  • Apollo asteroids have a semimajor axis greater than 1.0 AU and a perihelion less than Earth’s aphelion of 1.017 AU. The first known Apollo was 1862 Apollo, for which the group is named.
  • Aten asteroids have a semimajor axis less than 1.0 AU and an aphelion greater than Earth’s perihelion of 0.983 AU. The first known Aten was 2062 Aten, for which the group is named.

Less Technical Definition

  • Apollo asteroids spend most of their time beyond Earth’s orbit, but cross inside at some point.
  • Aten asteroids spend most of their time inside Earth’s orbit, but cross outside at some point.


The important thing to know is that both Apollos and Atens cross Earth’s orbit at some point. Keep in mind that space is three-dimensional, so their paths don’t necessarily intersect with Earth’s. They might pass “above” or “below” Earth, so to speak.

But the orbits of enough Apollos and Atens do intersect with Earth’s orbital path that they might one day hit us. Atens are particularly worrisome. They spend so much time inside Earth’s orbit, in relatively close proximity to the Sun, that it’s hard for astronomers to find them.

So if a giant asteroid ever does sneak up on us and wipe out human civilization, my guess is it’ll be an asteroid from the Aten group. Those are the asteroids that frighten me the most.


Don’t Panic: It’s Just Another Asteroid

November 23, 2016

People ask me all the time: “Hey, did you hear about that asteroid?” These people then tell me about some asteroid that’s supposed to “just barely miss us” is the next day or so. Sometimes, they also ask, “Aren’t you worried?”

There are certain kinds of space news that I simply can’t get excited about anymore. This is one of them. Why?


There’s actually a newsletter about asteroid flybys. It’s called Daily Minor Planet, and I have a subscription (it’s free). Every day in my inbox, I’m notified of the latest asteroid or other object skimming past Earth. Every day. Sometimes there are more than one per day.

Occasionally, one of these objects will pass within the radius of the Moon’s orbit. That’s not an everyday thing, but still… it happens more often than you might think.

So when people ask if I’ve heard about the latest asteroid flying past Earth, the only thing I can really say is, “Which one?” And if someone asks me if I’m worried, my answer is no. The asteroids that make headlines on the news and the asteroids that appear in Daily Minor Planet… those are asteroids we know about. It’s all the asteroids we don’t know about that scare the bejesus out of me.

Sciency Words: Shadow Biosphere

October 14, 2016

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:


Crazy Talk

We are not alone on planet Earth. There are aliens among us. Their existence has gone unnoticed and unsuspected for millions of years.

Truth be told, I shouldn’t call them “aliens.” They evolved here on Earth, side by side with what we, in our arrogance, call “organic life.”

They’re everywhere. There’s a whole biosphere of these weird creatures sharing our planet with us. It’s called the shadow biosphere.

Not Crazy Talk

I first heard about the shadow biosphere on an episode of SciShow, and I’ve been seeing the term more and more lately. It seems like some sort of astrobiology buzzword at the moment.

The idea is that an alternative form of life could have evolved here on Earth, and we just haven’t discovered it yet. Maybe it lives in areas totally inaccessible to us, like deep beneath the Earth’s crust. Or maybe it’s so different from us that we don’t yet recognize it as a life form.

Personally, I take this as more of an astrobiology thought experiment than a serious hypothesis about life on our planet. It’s a way of reminding us how limited our understanding of life is and show how difficult it might be to identify alien life should we happen to find it.

You see, to determine if something is alive, we must try to identify ways in which it is similar to other living things. Does it move? Grow? Reproduce? On a more fundamental level, is it cellular in structure? Does it have a carbon-based biochemistry? A DNA-like genetic code?


Little did the humans suspect that their “pet rocks” were in fact silicon-based life forms.

But all these questions presuppose that newly discovered life forms will be similar to life forms we already know about. What if we’re dealing with a life form totally dissimilar to life as we know it? What if they’re non-cellular, non-carbon-based organisms that don’t have anything resembling DNA?

Why, such organisms might be so strange to us that they could exist all around us, even right here on Earth, and we wouldn’t know it. Or so this type of thought experiment may lead you to conclude.

Back to Crazy Talk

It’s not just a thought experiment. The shadow biosphere is real. It’s real, I tell you! Wait, where are you taking me? No, I don’t want to take my medicine. Are you working for them? Did the pet rocks send you?

Earth vs. Asteroids

May 25, 2016

My25 Earth vs Asteroids

Is your planet safe? Nobody wants another Tunguska Event. Certainly we don’t want another K-T Event. So what are we doing to protect ourselves?

Thanks to grant money from NASA, the University of Hawaii has started setting up a series of telescopes specially designed to hunt for Near Earth Objects (N.E.O.s). These are objects, such as asteroids, with orbital paths that approach or cross Earth’s orbit.

The University of Hawaii’s new telescopes are collectively known as ATLAS (Asteroid Terrestrial-impact Last Alert System). Once fully operational, ATLAS promises to provide us with several weeks notice for large, incoming asteroids. For smaller, though still hazardous asteroids, ATLAS should give us at least a few hours warning.

So is our planet safe?

As more and more programs like ATLAS come online, we should get better coverage of the night sky and longer warning times for incoming objects. But there will still be a problem—a huge problem.

You see, you can’t really do astronomy during the day. That means asteroids coming at us from a sunward direction will go completely undetected. You may remember back in 2013 when a significantly-sized asteroid exploded over Russia, shattering windows and injuring over a thousand people. That asteroid came from a sunward direction.

No amount of ground-based telescopes could have detected that 2013 asteroid. But perhaps a space telescope, similar to the SOHO telescope we use to monitor solar flares, could help plug this gap in our planet’s defenses.

Sciency Words: The K-T Event

May 20, 2016

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:


You already know this story. It was 65 million years ago. There were dinosaurs, there was an asteroid…

It’s easily the most famous asteroid impact in Earth’s history, and it’s called the K-T Event, or sometimes the K-Pg Event.

In geology shorthand, the letters stand for:

  • K: the Cretaceous period, which is spelled with a K in German. This was the last period of geological history in which dinosaurs roamed the Earth.
  • T: the Tertiary period, which immediately followed the Cretaceous. According to the International Commission on Stratigraphy (ICS), we’re not supposed to use this name anymore, but people still do. It’s sort of like how some people keep calling Pluto a planet, no matter what the International Astronomy Union (IAU) says.
  • Pg: the Paleogene period, which is the period immediately following the Cretaceous according to the ICS’s new list of geological periods. Please note, the Tertiary and Paleogene are not really interchangeable terms. They have the same starting point, but different end points.

Geologists and paleontologists puzzled for decades over a layer of clay separating Cretaceous and Tertiary (or Paleogene) rock. They called it the K-T boundary. There were several competing hypotheses about what might have caused this boundary and how it related to the mass extinction event that killed off the dinosaurs.

Then in 1980, a paper came out entitled “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” This paper reported the discovery that the K-T boundary contained abnormally high levels of the element iridium.

Platinum group metals like iridium are extremely rare on Earth (except in the planet’s core) but common in asteroids. So whenever you find lots of iridium in Earth’s crust, you can justifiably assume an asteroid put it there.

The most likely scenario is that a large asteroid, about 10 km in diameter, smashed into Earth, flinging dust and debris high into Earth’s atmosphere. Enough to block out the sun worldwide for several years. This global dust cloud would have included plenty of material from the asteroid itself, which would have been partially vaporized by the heat of the impact.

A major problem with the original 1980 paper was that, at the time, no known impact crater of the appropriate age was sufficiently large. But of course, that was back in 1980. The crater has since been found in the Yucatan Peninsula, and now just about everybody knows the story of the K-T Event (even if they don’t know it’s called that).

P.S.: The K-T Event is not to be confused with the Katie Event. You know, that time your BFF Katie had waaaaay too much to drink and threw a temper tantrum of apocalyptic proportions.

Addendum: While there does seem to be general, widespread consensus that the K-T asteroid impact either caused the extinction of the dinosaurs or contributed significantly to their demise, there is not universal agreement. As Planetary Defense Commander notes in the comments, there are other possibilities worth considering.

I Spy with Hubble’s Eye…

February 1, 2016

Fun fact: the Hubble Space Telescope has observed and photographed every planet in the Solar System except Mercury. Mercury’s just too close to the Sun. I’m sure you know what a magnifying glass can do to ants, so imagine what would happen if we pointed Hubble—with all its oversized lenses and mirrors—in a sunward direction.

So does that mean Hubble has photographed Earth? The answer is yes, sort of.

Fb01 Earth Picture Day

A Hubble image of Earth would not look like this. Hubble orbits at a distance of only 550 kilometers (340 miles). That’s too close, so the image would look more like this.

Fb01 Earth Awkward Close Up

No, that’s still not close enough. Also, Hubble is moving at a velocity of over 25,000 kilometers per hour (16,000 miles per hour). So our snapshot of Earth turns out something like this.

Fb01 Earth Blur

But that doesn’t stop Hubble from snapping blurry photos of Earth anyway. Believe it or not, they’re useful to scientists. As the always brilliant Phil Plait explains (click here), the Hubble team uses these pictures, called flat-field images, to test and calibrate Hubble’s cameras.