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Excited Reports of 'Habitable Planets' Need to Come Back Down to Earth

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

In 1950, physicist Enrico Fermi famously asked, "Where are they?" as a kind of lament about the lack of observational evidence for alien intelligence in our universe. Today, the question is still asked in the context of the always-hoped-for discovery of other worlds like our own, with the thought that maybe, just maybe, we will finally find those aliens.

Against this backdrop, advances that have occurred in the past 20 years in the field of exoplanet discovery have excited the imaginations of scientists and enthusiasts alike.

When, the question goes, will we finally discover a planet that can sustain life? When will we discover Earth 2.0?

The impatience associated with this question has led many in the media and even some in the scientific community to make premature declarations that an "Earth analogue" has been discovered. But when exoplanets are discovered, claims that they are similar to Earth are based on, at best, optimistic modelling and, at worst, sensationalism.

Many such claims have been made on the basis of invented ranking systems that use the observed properties of exoplanets to extrapolate how Earth-like a planet might be. Unfortunately, these systems must make extremely simple – and almost certainly incorrect – assumptions about the characteristics of the planets they are trying to describe.

Ranking habitability: not so easy

Before any exoplanets had been discovered at all, certain astrophysicists proposed that each star had an associated zone around it that came to be known as the "habitable zone." This zone is at a distance from every star where a hypothetical Earth twin would have an average surface temperature between the freezing and boiling point of water. Too close and you exceed 100°C; too far and you drop below 0°C.

But if a planet has an atmospheric composition different from Earth’s, its true surface temperature is likely to be completely different.

Gaseous planets do not even have a well-defined surface to consider. And, for rocky planets, a thinner atmosphere can make them much colder (especially at night) while a thicker atmosphere can make them much hotter.

One of the most dramatic examples of this problem is Venus. Owing to its thick atmosphere and a runaway greenhouse effect, the planet has a temperature of a whopping 450°C — far higher than than the 25°C you would calculate given an Earth-like atmosphere. Although Venus lies within our Sun’s nominal habitable zone, it is surely not accurate to call it habitable.

The two most fruitful methods for discovering exoplanets (the "transit method" and the "radial velocity method") both give a straightforward way to determine the distance between a star and an exoplanet. That, together with our knowledge of how much heat is given off by the star, lets us calculate whether the planet is in the star's habitable zone. But, as we have seen, that is not the same thing as discovering a habitable planet.

Nevertheless, discoveries of planets in the habitable zones of other stars have been identified in the media and even in press releases of scientific institutions as discoveries of second Earths. Since we do not know the surface temperatures of any exoplanet, whether they are actual Earth analogues can only be guessed at using other lines of evidence.

Learning more about exoplanets

Our best understanding to date is that surface temperatures are strongly dependent on atmospheric composition and planet density. The density of a planet is dependent on both its mass and volume, but the two detection methods only allow for one or the other of the two complementary characteristics to be directly measured.

The transit method detects the shadow a planet casts on the star it is orbiting, allowing the planet's area (and, because planets are spheres, its volume) to be measured. What is not directly discoverable from this method, however, is the planet's mass.

Alternatively, the radial velocity method detects a planet via a wobble in the star's motion that can be used to infer a minimum possible planetary mass tugging on the star with its gravity. In many cases, the tug is being done at an angle so we see a reduced effect, which makes us infer a mass that is smaller than the actual mass of planet. Aside from this potential confusion, there is no way via the radial velocity method alone to determine a planet's volume.

Astrophysicists who model planet formation and composition have proposed a variety of models that offer possible relationships between the volumes and masses of planets depending on planet compositions.

The smallest planets in our own solar system are rocky and the largest planets are gaseous, but we see a number of exoplanets whose sizes lie between the smallest gaseous planet (Neptune) and the largest rocky planet (Earth). We have models that can accommodate "super-Earths" that are rocky or "mini-Neptunes" that are gaseous and all manner of hybrids in between.

These varied models can accommodate a range of atmospheres, and the exoplanets will have very different surface temperatures depending on all of this. It is therefore of some importance that we learn more about exoplanet atmospheres directly using better telescopes and more sensitive techniques.

Some astronomers have proposed a scheme to decide which exoplanets are most likely to have their atmospheres directly detected – the obvious next step in working towards determining a planet's surface temperature and ultimately whether it is habitable.

Jumping the gun

A regrettable tendency right now is to jump the gun. Planets have been discovered in the habitable zones of other stars with radial-velocity-measured minimum masses that are similar to Earth's and transit-measured surface areas that are not much larger than Earth's.

Crucially, none of these "analogues" has yet been measured in both ways. But almost every time such planets are discovered, breathless reports of their possible import are generated.

While discoveries of exoplanets are exciting, it is definitely premature to try to decide how Earth-like any planet is or is not on the basis of the scant data we are now able to gather. The best we can hope to do at this time is collate a list of possible targets for future observation.

Someday, we may discover definitive proof that another Earth is out there. But that day has not yet arrived – despite the excited headlines.

Joshua Tan, Astronomer at the Instituto de Astrofísica, Universidad Católica de Chile

This article was originally published on The Conversation. Read the original article. Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google +. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com.

For all we know, aliens from advanced civilizations at cosmological distances may be trying to send us messages. These signals, called fast radio bursts(FRBs), travel from far-away galaxies with no known source of origin, although some have speculated that they may come from extragalactic civilizations.

In light of the fact that soon, our cell phones could be used to measure and understand what these weird signals are, it’s worth taking a deeper dive into what exactly FRBs are, and why they matter.

FRBs are intense signals that are similar to radio or laser signals. They might not be a force of nature, but possibly artificial in origin. Researchers are puzzled about where they came from, and in the past decade, less than two dozen FRBs have been identified using giant radio telescopes like the one in Arecibo Observatory in Puerto Rico.

“They’re extremely bright and their nature is not understood,” Avi Loeb of the Harvard-Smithsonian Center for Astrophysics, whose new paper details how we might be able to use cellular phones to better study FRBs, tells Inverse. “They require coherent emission by many electrons, of the type that you get in a radio antenna or a laser.”

There might be FRBs alive right in our Milky Way though. These signals are louder than the ones from distant galaxies, and using cell phones, we may find out where they actually come from — extraterrestrial life, nature, or something else.

“We might discover that one of these ‘monster FRBs’ lives in our galaxy, the Milky Way, in our own ‘backyard’, so it is ten billion times louder than the very distant FRBs we have seen so far,” study coauthor Dan Maoz of Tel Aviv University tells Inverse.

Although currently we have giant radio telescopes detecting FRBs, most FRBs last for a thousandth of a second and originate from the edge of the universe. A signal in the Milky Way would be much louder.

“Because an FRB from the Milky Way should be so much brighter, it can be detected with a cell phone or a small electronic device that costs tens of dollars instead of a giant radio dish,” Loeb says.

“Large radio telescopes can point at a given time only to a very small part of the sky, typically the size of the full moon, and they are designed to detect signals billions of time fainter than the nearby FRBs we are thinking of, which are comparatively very bright, while no one can say where in the sky this FRB will be,” Maoz says. “The network of cellphones is ideal both in terms of being able to receive the signal and in terms of being sensitive to signals from any direction.”

With the help of citizen scientists, scientists can study FRBs in different frequencies of radiation, like radio waves, visible light, and X-rays. One possibility is using a Citizens-Science app, where people with the app can listen for and record candidate FRBs and upload the recordings to a website. Another option is using FM-radio receivers in cell phones to search for FRBs at lower frequencies. A third option is using “software-defined radio” devices that cost about $10. These can be plugged into USB ports of personal computers to create a global network of receivers. A global network of these detectors could find nearby FRBs in no time.

Researchers believe that if an FRB occurs in our own galaxy, a global network of cell phones or radio receivers can “hear” it, especially since previous FRBs were detected at radio frequencies that match those used by wifi and cell phones. So if an alien from an extragalactic civilization calls us, we just need to pick up our phones.

Abstract:

If fast radio bursts (FRBs) originate from galaxies at cosmological distances, then their all-sky rate implies that the Milky Way may host an FRB on average once every 30-1500 years. If FRBs repeat for decades or centuries, a local FRB could be active now. A typical Galactic FRB would produce a millisecond radio pulse with ~1 GHz flux density of ~3E10 Jy, comparable to the radio flux levels and frequencies of cellular communication devices (cell phones, Wi-Fi, GPS). We propose to search for Galactic FRBs using a global array of low-cost radio receivers. One possibility is to use the ~1GHz communication channel in cellular phones through a Citizens-Science downloadable application. Participating phones would continuously listen for and record candidate FRBs and would periodically upload information to a central data processing website, which correlates the incoming data from all participants, to identify the signature of a real, globe-encompassing, FRB from an astronomical distance. Triangulation of the GPS-based pulse arrival times reported from different locations will provide the FRB sky position, potentially to arc-second accuracy. Pulse arrival times from phones operating at diverse frequencies, or from an on-device de-dispersion search, will yield the dispersion measure (DM) which will indicate the FRB source distance within the Galaxy. A variant of this approach would be to use the built-in ~100 MHz FM-radio receivers present in cell phones for an FRB search at lower frequencies. Alternatively, numerous “software-defined radio” (SDR) devices, costing ~$10 US each, could be plugged into USB ports of personal computers around the world (particularly in radio quiet regions) to establish the global network of receivers.

Photos via Flickr / Keoni Cabral