Brian Cox on the planets of Holst at the time and now | The music

When The Planets was completed in 1916, little was known about the physical nature of the musically represented worlds of Gustav Holst, and he did not care. His focus was on the planets as metaphors for different facets of the psyche; War, Peace, Jollity, Old Age, Messenger, Magician and Mystic. Indeed, Holst wrote parts of the work as stand-alone pieces and co-opted them later.

Today, we have visited all the planets and our discoveries have replaced their ancient astrological characters. At first glance, this new knowledge may seem to contradict Holst's work, but it would be a superficial conclusion to draw. The planets have a much richer story than Holst could have imagined, and reality delivers more powerful metaphors than myth. Compared to what we know now, Holst's work catalyzes new ideas and generates powerful intellectual challenges that enrich and inform the important debates underway today, as deep art can and can do.

The music of the past is not a fossil in the same way that Aristotle, Shakespeare and Newton do not belong to the airless cases; music emerges from the ferment of ideas and contributes to the vitality of beer; as such, it is a transcendent ingredient of intellectual life.

Perhaps the most striking example is Venus; Bringer of Peace, the evening star, the brightest of all worlds. In the early 1960s, well-known astronomers imagined a tropical paradise under its ever-present cloud cover, but Russian and American spacecraft destroyed those hopes towards the end of the decade. The planet's surface temperatures exceed 460 ° C, and the rocks are crushed by atmospheric pressure a hundred times more than the Earth and bathed by the rains of sulfuric acid. Venus is a vision of hell.

Yet Venus may have been once tempered, perhaps supporting life, with rivers and oceans and a blue sky. Two billion years ago, while life was well established on Earth, a blazing greenhouse effect boiled the oceans and thickened the atmosphere, destroying any eventual life. Listen to Holst's Venus, keeping this story in mind, and the song becomes a requiem for a failing planet, a paean with transience. The laws of nature allow the spontaneous emergence of beauty but only as an intermediate destination between creation and decline. Remember that the planets, just like human beings, are born and will die one day, and it is this fragility and this inevitable transitory that make them precious.

March, Bringer of War, red blood in the imagination, has always been a mirror for our nightmares and dreams. Written before the First World War, Holst's motivation was probably a critique of industrial capitalism; a prophetic work that found new resonance and power in the face of mechanized war images that dominated the decades that followed its composition.

Before flying over the Mariner 4 spacecraft in July 1965, it was possible to imagine Mars as a living world; a new Eden perhaps, or the envious eyes of the HG Wells invaders. Mariner's photographs revealed a very different Mars; a dry world no different from our Moon. It was a shock and pushed President Lyndon Johnson to a rare and precious moment of existential thought – for a politician. "Perhaps, it is perhaps just that life as we know it, with its humanity, is more unique than many thought."The hesitation in the first words is revealing. Here, Mars is a symbol of our cosmic isolation. It's as if, deep within the subconscious, the power brokers of the 1960s until the president had suddenly realized that the Earth was much more fragile and valuable than an objective analysis of their cold war could suggest. Or maybe the prospect of exploration is always shocking. Earthrise of Apollo 8, the photograph that gave such a positive end to a troubled 1968 year by placing the Blue Earth against the Gray Moon, was in three years, but the Mars red offered a foretaste.

More recent missions on Mars have revealed a more nuanced story. Mars was certainly warmer and wetter 3.5 billion years ago as life grew on Earth. This means that life may have started on the red planet and may still exist in the underground water reservoirs we know today. The discovery of a second genesis in our solar system would have profound philosophical, scientific and cultural consequences, because it would mean that the origin of life is inevitable. And that would mean that the universe is most likely to live: that we are not alone.

A planet with a history like Mars could also play a determining role in our future. Mars is rich in resources. all things necessary to support a civilization. Even if there are no Martians today, there will be soon: The Martians will be us. Mars is the only planet we could hope to visit. The others are far too hostile.

Mars is not a war bearer; it's Bringer of Hope. Is it too much to ask to listen to the Mars planet of Holst and to forget the deeply rooted symbolism of the twentieth century? To engage in a debate with the composer in the 21st century, after surviving barbarism, our technology and rockets are released to offer a perspective on our cosmic isolation and serve as a bridge to a future where humanity will no longer be confined to a only world? "The Earth is the cradle of humanity," wrote the great Russian scientist Konstantin Tsiolkovsky, "but we can not stay in the cradle forever".

The planets end with Neptune the mystic. Unlike Mars and Venus, we do not need to work so hard to find meaning in the synthesis of music and ideas. There is only one photograph taken by Voyager 2 in 1989 that improves music more than words. It's my personal favorite of the long and spectacular history of space exploration. Here is a picture of the frozen outpost of the solar system at over four billion kilometers from Earth. Crescent Neptune and its moon Triton, a world of ice at 230 degrees below zero revolving around a blue planet of storms, yet incredibly geologically active with cryovolcans that sprinkle nitrogen with snow on the surface.

Two worlds unpublished for 4.5 billion years after their formation until a tiny emissary of the Earth passes into interstellar space. A twilight catwalk to the stars that remains in the imagination as the choir faints.

• Professor Brian Cox joins the BBC Symphony Orchestra for The Planets of Holst at the Barbican, London, on September 29, broadcast live on Radio 3 and available on television as part of "Our Classical Century" year next.