A"gravitational telescope" capable of showing the morphology and surface of exoplanets.
In recent decades thousands of new planets have been discovered beyond our Solar System,
the so-called exoplanets. Objects orbiting other stars that obviously, at best, have
only been observed as small and simple dots. However, in the next few years, it is very
likely that we will be able to observe the morphology and some characteristics of the
surface, such as the presence of vegetation, mountains, or even signs of habitability.
If this will happen it will be thanks to a new project funded by NASA that plans to make
a "natural" telescope, based on the property of light to be focused by means of a large
mass that acts as a "gravitational lens". OK, let's figure this out!
The search for an Earth-like planet orbiting another star is one of astronomy's greatest
challenges. It's a task that appears close to fruition. Since astronomers spotted the
first exoplanet in 1988, they have found more than 4000 others.
Most of these planets are huge because bigger objects are easier to spot. But as sensing
techniques and technologies improve, astronomers are finding planets that match Earth's vital
statistics ever more closely. They have even begun to use a ranking system
called the Earth Similarity Index to quantify how similar an exoplanet is to the mother
planet. The exoplanet that currently ranks most highly is Kepler-438b, which orbits in
the habitable zone of a red dwarf in the constellation of Lyra some 470 light years from here.
Kepler-438b has an Earth Similarity Index of 0.88. By comparison, Mars has an ESI of
0.797, so it's more Earth-like than our nearest neighbor. That's exciting but it is inevitable
that astronomers will find planets with even higher indices in the near future.
And that raises an interesting question: how much can we ever know about these planets,
given their size and distance from us? To understand the magnitude of the problem,
let's imagine we want to observe the Earth from a distance of 100 light years.
From such a distance, our planet, which has a diameter of about 12,750 km, subtends an
arc of just 3 millionths of an arcsecond: an angle 17,600 times smaller than the smallest
detail that a large telescope like Hubble, for example, is able to resolve and show us.
And this is without considering that, from such a distance, the weak light reflected
from Earth would be completely drowned in the overwhelming glare of the Sun.
If we wanted to be able to see the Earth as a disc, and not as a simple point of light,
we would need a telescope with a mirror not 80 meters in diameter, already huge for our
construction standards, but of 80 km. If we then wanted to obtain a detailed image with
a resolution capable of showing us seas and continents, we would need a telescope with
an aperture of about 50,000 km, that is four times larger than the diameter of the Earth!
What should we do, then? Give up forever the possibility to have a look at the conformation
of an alien planet? Maybe yes, maybe no...
Imagine having an instrument on extrasolar planets that would allow you to observe not
only the shapes of their continents and oceans, if they have any, but details just a few kilometers
large... It would be a revolution not only technological but also cultural: it would
make very clear to us the concept that the Earth is only one of an infinite number of
worlds. Well, perhaps you will be surprised to know
that this tool is already feasible, with existing technologies or their refinements: it is called
Solar Gravity Lens, and works using a fascinating technique. Its primary lens, in fact, is an
entire star. In this case, our Sun!
It is not pure theory: astronomers have already observed this phenomenon in concrete images
of distant galaxies, whose appearance is deformed because their light comes to us passing in
the vicinity of another celestial body of large mass (for example, another galaxy relatively
closer). The fact that a concentrated mass is able
to deviate in a measurable way the path of light is one of the cornerstones of general
relativity and the principle at the base of the functioning of gravitational lenses. We
must always remember that Einstein became world-famous only in 1919, when the English
scientist Arthur Eddington was able to demonstrate, thanks to photographs taken during a total
eclipse of the Sun, that the theory of relativity correctly predicted the small shift, caused
by the mass of the Sun, of the position of some stars randomly aligned with the solar
edge. But the Sun's mass can do more than slightly
deflect the path of light rays passing in its vicinity. If a light source is located
exactly behind the Sun with respect to the position of an observer, then, looking in
the direction of the Sun from a sufficient distance, the observer will see around the
solar disk a ring-like structure that contains the image (distorted and magnified) of that
source: it is the so-called Einstein ring, the most perfect example of gravitational
lens.
Thanks to this optical phenomenon, it is possible to observe objects that would otherwise be
invisible even for the most powerful telescopes. The mass acting as a lens produces, in fact,
two extraordinarily favorable effects: it magnifies the remote source randomly aligned
with the observer and greatly enhances the light intensity.
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It should be immediately clear that it is not at all easy to succeed in using a gravitational
lens for such a purpose, but it is possible. And the fact that it is possible is evidenced
by the only project that has reached phase 3 of the 2020 selection of the NASA NIAC program.
NIAC stands for NASA Innovative Advanced Concepts and, as the name suggests, it's an initiative
that rewards innovative projects for future space missions, offering more and more funding
as a selected project passes the preliminary stages in which its feasibility and the degree
of completeness of the idea is evaluated. The project now in Phase 3 of this year's
edition is called Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar
Gravitational Lens Mission. Its promoters are Slava Turyshev, a Russian physicist working
at NASA Jet Propulsion Laboratory, and Viktor Toth, a software developer who has advanced
knowledge of physics and is the author, often together with Turyshev, of numerous studies
published on some of the major scientific journals.
The object that ends up under the gravitational lens is magnified to the point that a telescope
with a 1-meter primary mirror could reach, observing the image produced by the lens,
an angular resolution of 1 billionth of an arc second. With such a resolution, details
on the order of kilometers could be resolved on the surface of an exoplanet up to 100 light
years from Earth. It would be possible, for example, to see the lights in the night hemisphere
of the planet, if it was inhabited by a technological civilization capable of illuminating the night,
covering vast territories with artificial lights as we humans do.
Why this extraordinary possibility of knowledge has not been exploited before? For at least
two good reasons. The first is the difficulty to reconstruct the image of the exoplanet
from the Einstein ring; the second has to do with the distance from the Sun, from which
begins to be visible the Einstein ring that contains the image of the exoplanet.
Let's start from the first point. Gravitational lenses are not perfect lenses. The images
they produce, although exceptional from the point of view of magnification and enhancement
of light intensity of the source, are severely distorted due to an optical phenomenon called
spherical aberration. As a result of it, the light rays from the remote source are bent
at a variable angle, which depends on the distance at which each ray passes relative
to the mass acting as a lens. Because of spherical aberration, there is not a single focal point,
where all the light rays converge, but a focal line, which continues indefinitely as one
moves away from the mass acting as a lens. Because of spherical aberration, each point
of the Einstein ring formed by light from an exoplanet aligned with the Sun, and the
observer contains a superposition of images. A complex mathematical analysis of how light
rays from different parts of the planetary surface interact with each other is therefore
required to hope to faithfully reconstruct a sharp image of the exoplanet observed through
the gravitational solar lens. To make things more complicated, there is
the not secondary problem that the source focused by the gravitational lens continuously
changes its appearance. An exoplanet observed through an Einstein ring is in fact not an
immobile object. It orbits around its star and rotates around its own axis. This means
that the portion of the planetary surface observed at any point along the focal line
created by the gravitational lens changes very rapidly.
The rapid variability is a consequence of the fact that the very high resolution of
the gravitational lens allows to observe extremely small regions of the light source, a few tens
of kilometers long at most. It is like having a very small window, inside which portions
of the surface of an object flowing continuously. Taking into account all these factors, and
others, the reconstruction of a detailed image of an exoplanet observed through the solar
gravitational lens will inevitably be a long and extremely laborious operation. First of
all, the space telescopes (if they will be more than one) sent in the area where the
Einstein ring will be visible will have to find the exact place where the image of the
exoplanet is formed.
The second major obstacle to the use of the solar gravitational lens is the distance to
which one must travel to reach the "focal point" of the whole system. Given the mass
and radius of the Sun, the minimum distance at which the ring becomes visible is about
550 astronomical units (the astronomical unit is the average distance between the Earth
and the Sun, and corresponds to just under 150 million kilometers), equal to 82 billion
kilometers. It is an immense distance, to which no probe
built by mankind has so far never reached. The Voyager 1, launched in 1977, is the most
distant human artifact to date. After 44 years of travel, it arrived at 23 billion kilometers
from the Sun: only a quarter of the distance that must be covered to arrive in the area
useful to exploit the solar gravitational lens.
It is understood from these data that the probe or the group of robotic probes that
will have the task of observing the solar gravitational lens will have to possess a
much higher speed: more than 5 times greater than that with which Voyager 1 is moving away
from the Sun. But it is probably not a good idea to invest
years of planning on a single heavy and expensive probe. A wiser option would be to launch a
whole fleet of very light and inexpensive robotic probes, propelled by solar sails.
What would be lost in quality and power of the scientific instrumentation at single craft
level, could be regained with interest through the redundancy of observations, guaranteed
by the availability of a large number of space telescopes, all engaged in the same task,
coordinated by an artificial intelligence that learns by trial and error.
Among other things, a spacecraft equipped with solar sails would continue to accelerate
all the time instead of having only the brief initial cue of traditional rockets. The result
is that a solar sail carrying a telescope could reach the Sun's focal point in about
twenty-five years. Arriving at the right distance and in the
right position, the telescope would point its sensors in the direction of the Sun, equipped
with a coronagraph capable of blocking light from the solar disk.
This would allow the telescope to capture the image of an exoplanet, distorted in an
Einstein ring, placing itself so as to have the Sun exactly aligned with that exoplanet.
In reality, the ring would be double: one would contain light from a single area about
10 km in diameter of the exoplanet, while the other would contain light from the rest
of the alien world. Slightly moving the telescope about a kilometer in different directions
would change the area of the exoplanet "framed" by the first ring and then you could do a
slow scan of its entire surface. Observing these distortions for six months and processing
about a million images collected, it would be possible to exclude the light of the solar
corona and obtain an image similar to that (simulated) shown at the beginning of this
article, even eliminating any clouds. In addition to the image of the surface, this
telescope would also do spectroscopy of the exoplanet, allowing to know the chemical composition
of its eventual atmosphere. If an alien civilization were to do this with us, pointing a solar
gravitational lensing telescope at Earth, it could detect the sudden increase in atmospheric
CO2 and other pollutants and infer the presence of industrial activities by the planet's unwise
inhabitants. Building a gravitational telescope is, in
short, a remarkable engineering challenge, involving exceptional navigational precision
and unprecedented radio communication difficulties, but requiring nothing we do not already know.
A fleet of these telescopes, placed in various points of the circumference surrounding the
Sun at a distance of 550 astronomical units, could observe in a few decades all the planets
located less than 100 light years from Earth. If all this seems too sci-fi, and if the idea
of being able to see the continents of distant worlds seems unfeasible, consider what I said
before ... and that is that NASA is so interested in this concept to fund research on its feasibility
with two million dollars. And if they believe in it, why shouldn't we believe in it?